2021

Journal articles 

Alexander Kröller,
Falk Hüffner,
Łukasz Kosma,
Katja Kröller, and
Mattia Zeni:
Driver expectations toward strategic routing.
Transportation Research Record 2675(11):44–53, 2021.
[show abstract]
Abstract.
Strategic Routing is a traffic intervention mechanism. To reduce
traffic in a certain area, drivers are asked to take a
predefined route diverting them from the area, even though this
increases their travel time. This can be implemented with
navigation apps or indash navigation from navigation service
providers such as TomTom or Google. Triggered by a traffic
authority, the driver receives information through the service provider’s
infrastructure and user interface about a new route and
potentially a reward.
In this work, we investigate the dynamics between traffic
authority, service provider, and end user by analyzing user
expectations. Ultimately, service providers are competing for
customers. They can, therefore, only imple ment strategic routing
if it appeals to drivers rather than scares them away. We
report on the insights of a twostage study with 457
participants, exploring what kind of strategic routing
interventions are appreciated by drivers.
We find that drivers report a high interest in seeing diversion
suggestions, even when they are not inclined to take
them. However, they are unwilling to have their route adapted
automatically. Important factors affecting drivers’ willingness to divert are
the reason for the detour and the additional driving
time. Incentives do increase the efficacy, but only marginally
increase user appreciation, indicating that users may
mistrust strategic routing that relies too strongly on
incentives.

2020

Conference articles 

Thomas Bläsius,
Maximilian Böther,
Philipp Fischbeck,
Tobias Friedrich,
Alina Gries,
Falk Hüffner,
Otto Kißig,
Pascal Lenzner,
Louise Molitor,
Leon Schiller,
Armin Wells, and
Simon Wietheger:
A strategic routing framework and algorithms for computing alternative paths.
In Proceedings of the 20th Symposium on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems
(ATMOS ’20). September 2020.
Volume 85 in OpenAccess Series in Informatics (OASIcs), pages 10:1–10:14, Schloss Dagstuhl–LeibnizZentrum für Informatik.
[show abstract]
Abstract.
Traditional navigation services find the fastest route for a
single driver. Though always using the fastest route seems
desirable for every individual, selfish behavior can have
undesirable effects such as higher energy consumption and
avoidable congestion, even leading to higher overall and
individual travel times. In contrast, strategic routing aims at
optimizing the traffic for all agents regarding a global
optimization goal. We introduce a framework to formalize
realworld strategic routing scenarios as algorithmic problems
and study one of them, which we call Single Alternative Path
(SAP), in detail. There, we are given an original route between
a single origin–destination pair. The goal is to suggest an
alternative route to all agents that optimizes the overall
travel time under the assumption that the agents distribute
among both routes according to a psychological model, for which
we introduce the concept of Paretoconformity. We show that the
SAP problem is NPcomplete, even for such models. Nonetheless,
assuming Paretoconformity, we give multiple algorithms for
different variants of SAP, using multicriteria shortest path
algorithms as subroutines. Moreover, we prove that several
natural models are in fact Paretoconform. The implementation of
our algorithms serves as a proof of concept, showing that SAP
can be solved in reasonable time even though the algorithms have
exponential running time in the worst case.

2019

Patents 

Timur Muraleev,
Falk Hüffner,
Alexander Kröller,
Neil Clavin,
Steffen Gunther Wiesner,
Massimo Guggino,
Michael Marte, and
Henning Hasemann:
Methods and systems for generating parking routes.
Patent WO/2019/180004, 2019.
[show abstract]
Abstract.
A method of determining a parking route for a vehicle travelling
on a road network within a geographic area is disclosed. A
subnetwork is determined comprising a subset of segments of an
electronic map that are representative of roads within a
predetermined walking time or distance of a destination
location. Data indicative of a walking time or distance from the
segment to the destination location is associated, at least with
the segments of the subnetwork representing roads having at
least one associated parking space. A search algorithm having an
associated cost function is used to explore the segments of the
subnetwork from an origin location to identify a plurality of
candidate parking routes, wherein the cost for a given parking
route is based on the probability of the vehicle successfully
finding a parking space on the parking route and the expected
cumulative travel and walking time or distance to the
destination location should a parking space be found along the
parking route.

2018

Journal articles 

Robert Bredereck,
Jiehua Chen,
Falk Hüffner, and
Stefan Kratsch:
Parameterized complexity of team formation in social networks.
Theoretical Computer Science 717:26–36, 2018
(original publication).
[show abstract]
Abstract.
Given a task that requires some skills and a social network of
individuals with different skills, the Team Formation problem
asks to find a team of individuals that together can perform the
task, while minimizing communication costs. Since the problem is
NPhard, we identify the source of intractability by analyzing
its parameterized complexity with respect to parameters such as
the total number of skills k, the team size l, the
communication cost budget b, and the maximum vertex
degree Δ. We show that the computational complexity
strongly depends on the communication cost measure: when using
the weight of a minimum spanning tree of the subgraph formed by
the selected team, we obtain fixedparameter tractability for
example with respect to the parameter k. In contrast,
when using the diameter as measure, the problem is intractable
with respect to any single parameter; however,
combining Δ with either b or l yields
fixedparameter tractability.

Guillaume Fertin,
Falk Hüffner,
Christian Komusiewicz, and
Manuel Sorge:
Matching algorithms for assigning orthologs after genome duplication events.
Computational Biology and Chemistry 74:379–390, 2018
(original publication).
[show abstract]
Abstract.
In this paper, we introduce and analyze two graphbased models
for assigning orthologs in the presence of wholegenome
duplications, using similarity information between pairs of
genes. The common feature of our two models is that genes of the
first genome may be assigned two orthologs from the second
genome, which has undergone a wholegenome
duplication. Additionally, our models incorporate the new notion
of duplication bonus, a parameter that reflects how assigning
two orthologs to a given gene should be rewarded or
penalized. Our work is mainly focused on developing exact and
reasonably timeconsuming algorithms for these two models: we
show that the first one is polynomialtime solvable, while the
second is NPhard. For the latter, we thus design two
fixedparameter algorithms, i.e. exact algorithms whose running
times are exponential only with respect to a small and
wellchosen input parameter. Finally, for both models, we
evaluate our algorithms on pairs of plant genomes. Our
experiments show that the NPhard model yields a better cluster
quality at the cost of lower coverage, due to the fact that our
instances cannot be completely solved by our
algorithms. However, our results are altogether encouraging and
show that our methods yield biologically significant predictions
of orthologs when the duplication bonus value is properly
chosen.

2017

Journal articles 

René van Bevern,
Robert Bredereck,
Morgan Chopin,
Sepp Hartung,
Falk Hüffner,
André Nichterlein, and
Ondřej Suchý:
Fixedparameter algorithms for DAG partitioning.
Discrete Applied Mathematics 220:134–160, 2017
(original publication).
[show abstract]
Abstract.
Finding the origin of short phrases propagating through the web
has been formalized by Leskovec et al. (2009)
as DAG Partitioning: given an
arcweighted directed acyclic graph on n vertices and m arcs,
delete arcs with total weight at most k such that each
resulting weaklyconnected component contains exactly one sink—a
vertex without outgoing arcs. DAG Partitioning is NPhard.
We show an algorithm to solve DAG Partitioning in
O(2^{k} · (n + m)) time,
that is, in linear time for fixed k. We complement it
with lineartime executable data reduction rules. Our
experiments show that, in combination, they can optimally
solve DAG Partitioning on simulated
citation networks within five minutes for k ≤ 190
and m being 10^{7} and larger. We use our
obtained optimal solutions to evaluate the solution quality of
Leskovec et al.’s heuristic. We show that Leskovec et al.’s
heuristic works optimally on trees and generalize this result by
showing that DAG Partitioning is
solvable in 2^{O(t2)}
· n time if a widtht tree decomposition of the
input graph is given. Thus, we improve an algorithm and answer
an open question of Alamdari and Mehrabian (2012). We
complement our algorithms by lower bounds on the running time of
exact algorithms and on the effectivity of data reduction.

Book chapters 

2016

Conference articles 

Robert Bredereck,
Jiehua Chen,
Falk Hüffner, and
Stefan Kratsch:
Parameterized complexity of team formation in social networks.
In Proceedings of the 11th International Conference on Algorithmic Aspects in Information and Management
(AAIM ’16),
Bergamo, Italy. July 2016.
Volume 9778 in Lecture Notes in Computer Science, pages 137–149,
Springer, 2016 (original publication).
Journal version available.
[show abstract]
Abstract.
Given a task that requires some skills and a social network of
individuals with different skills, the Team
Formation problem asks to find a team of individuals that
together can perform the task, while minimizing communication
costs. Since the problem is NPhard, we identify the source of
intractability by analyzing its parameterized complexity with
respect to parameters such as the total number of
skills k, the team size l, the communication cost
budget b, and the maximum vertex degree Δ. We show
that the computational complexity strongly depends on the
communication cost measure: when using the weight of a minimum
spanning tree of the subgraph formed by the selected team, we
obtain fixedparameter tractability for example with respect to
the parameter k. In contrast, when using the diameter as
measure, the problem is intractable with respect to any single
parameter; however, combining Δ with either b
or l yields fixedparameter tractability.

2015

Conference articles 

Falk Hüffner,
Christian Komusiewicz, and
André Nichterlein:
Editing graphs into few cliques: complexity, approximation, and kernelization schemes.
In Proceedings of the 14th Algorithms and Data Structures Symposium
(WADS ’15),
Victoria, Canada. August 2015.
Volume 9214 in Lecture Notes in Computer Science, pages 410–421,
Springer, 2015 (original publication).
[show abstract]
Abstract.
Given an undirected graph G and a positive
integer k, the Sparse Split Graph
Editing problem asks to transform G into a graph
that consists of a clique plus isolated vertices by performing
at most k edge insertions and deletions; similarly,
the P_{3}Bag Editing
problem asks to transform G into a graph which is the
union of two possibly overlapping cliques.
Sparse Split Graph Editing is
NPcomplete; we give a simple lineartime 3approximation
algorithm, an improvement over a more involved known
factor3.525 approximation. Further, we show
that P_{3}Bag Editing is
also NPcomplete. Finally, we present a kernelization scheme
for both problems and additionally for the
2Cluster Editing problem. This scheme
produces for each fixed ϵ in polynomial time a kernel of
size ϵk. This is, to the best of our knowledge, the first
example of a kernelization scheme that converges to a lower
bound.

Falk Hüffner,
Christian Komusiewicz, and
Manuel Sorge:
Finding highly connected subgraphs.
In Proceedings of the 41st International Conference on Current Trends in Theory and Practice of Computer Science
(SOFSEM ’15),
Pec pod Sněžkou, Czech Republic. January 2015.
Volume 8939 in Lecture Notes in Computer Science, pages 254–265,
Springer, 2015 (original publication).
[show abstract]
Abstract.
A popular way of formalizing clusters in networks are
highly connected subgraphs, that is, subgraphs
of k vertices that have edge connectivity larger
than k/2 (equivalently, minimum degree larger
than k/2). We examine the computational complexity of
finding highly connected subgraphs. We first observe that this
problem is NPhard. Thus, we explore possible parameterizations,
such as the solution size, number of vertices in the input, the
size of a vertex cover in the input, and the number of edges
outgoing from the solution (edge isolation), and expose their
influence on the complexity of this problem. For some
parameters, we find strong intractability results; among the
parameters yielding tractability, the edge isolation seems to
provide the best tradeoff between running time bounds and a
small parameter value in relevant instances.

Journal articles 

René van Bevern,
Jiehua Chen,
Falk Hüffner,
Stefan Kratsch,
Nimrod Talmon, and
Gerhard J. Woeginger:
Approximability and parameterized complexity of multicover by cintervals.
Information Processing Letters 115(10):744–749,
2015
(original publication).
[show abstract]
Abstract.
A cinterval is the disjoint union of c intervals
over ℕ. The cInterval Multicover
problem is the special case of Set
Multicover where all sets available for covering
are cintervals. We strengthen known APXhardness results
for cInterval Multicover, show
W[1]hardness when parameterized by the solution size, and
present fixedparameter algorithms for alternative
parameterizations.

Sharon Bruckner,
Falk Hüffner, and
Christian Komusiewicz:
A graph modification approach for finding core–periphery structures in protein interaction networks.
Algorithms for Molecular Biology 10:16,
2015.
[show abstract]
Abstract.
The core–periphery model for protein interaction (PPI) networks
assumes that protein complexes in these networks consist of a
dense core and a possibly sparse periphery that is adjacent to
vertices in the core of the complex. In this work, we aim at
uncovering a global core–periphery structure for a given PPI
network. We propose two exact graphtheoretic formulations for
this task, which aim to fit the input network to a hypothetical
ground truth network by a minimum number of edge
modifications. In one model each cluster has its own periphery,
and in the other the periphery is shared. We first analyze both
models from a theoretical point of view, showing their
NPhardness. Then, we devise efficient exact and heuristic
algorithms for both models and finally perform an evaluation on
subnetworks of the S. cerevisiae PPI network.

Falk Hüffner,
Christian Komusiewicz,
Rolf Niedermeier, and
Martin Rötzschke:
The parameterized complexity of the rainbow subgraph problem.
Algorithms 8(1):60–81, 2015
(original publication).
[show abstract]
Abstract.
The NPhard Rainbow Subgraph problem,
motivated from bioinformatics, is to find in an edgecolored
graph a subgraph that contains each edge color exactly once and
has at most k vertices. We examine the parameterized
complexity of Rainbow Subgraph for
paths, trees, and general graphs. We show
that Rainbow Subgraph is W[1]hard with
respect to the parameter k and also with respect to the
dual parameter l := n − k
where n is the number of vertices. Hence, we examine
parameter combinations and show, for example, a polynomialsize
problem kernel for the combined parameter l and “maximum
number of colors incident with any vertex”.

2014

Conference articles 

Sharon Bruckner,
Falk Hüffner, and
Christian Komusiewicz:
A graph modification approach for finding core–periphery structures in protein interaction networks.
In Proceedings of the 14th Workshop on Algorithms in Bioinformatics
(WABI ’14),
Wrocław, Poland. September 2014.
Volume 8701 in Lecture Notes in Bioinformatics, pages 340–351,
Springer, 2014 (original publication).
Journal version available.
[show abstract]
Abstract.
The core–periphery model for protein interaction (PPI) networks
assumes that protein complexes in these networks consist of a
dense core and a possibly sparse periphery that is adjacent to
vertices in the core of the complex. In this work, we aim at
uncovering a global core–periphery structure for a given PPI
network. We propose two exact graphtheoretic formulations for
this task, which aim to fit the input network to a hypothetical
ground truth network by a minimum number of edge
modifications. In one model each cluster has its own periphery,
and in the other the periphery is shared. We first analyze both
models from a theoretical point of view, showing their
NPhardness. Then, we devise efficient exact and heuristic
algorithms for both models and finally perform an evaluation on
subnetworks of the S. cerevisiae PPI network.

Falk Hüffner,
Christian Komusiewicz,
Rolf Niedermeier, and
Martin Rötzschke:
The parameterized complexity of the rainbow subgraph problem.
In Proceedings of the 40th International Workshop on GraphTheoretic Concepts in Computer Science
(WG ’14),
Le Domaine de Chalès, France, June 2014.
Volume 8747 in Lecture Notes in Computer Science, pages 287–298,
Springer, 2014 (original publication).
Journal version available.
[show abstract]
Abstract.
The Rainbow Subgraph problem, motivated
from bioinformatics, is to find in an edgecolored graph a
subgraph with a minimum number of vertices that contains all
colors. We examine the parameterized complexity
of Rainbow Subgraph for paths, trees,
and general graphs. We show APXhardness even on properly
edgecolored paths where every color occurs at most twice, and
that no single natural parameter yields tractability. Hence, we
examine parameter combinations and obtain e.g. tractability and
a kernel for the combination of “number of vertices not in the
solution” and “maximum number of colors incident on a vertex”.

Journal articles 

Laurent Bulteau,
Falk Hüffner,
Christian Komusiewicz, and
Rolf Niedermeier:
Multivariate algorithmics for NPhard string problems.
Bulletin of the EATCS 114:31–73, 2014
(original publication).
Note: Compared to the published version, this version
fixes typos in Figure 1 and in Challenge 13.
[show abstract]
Abstract.
String problems arise in various applications ranging from text
mining to biological sequence analysis. Although string problems
are often considered to be easier than graph problems, many
string problems are NPhard as well. This motivates the search
for (fixedparameter) tractable special cases of these
problems. We survey the state of the art concerning
parameterized algorithmics for NPhard string problems and
identify challenges for future research.

Falk Hüffner,
Christian Komusiewicz,
Adrian Liebtrau, and
Rolf Niedermeier:
Partitioning biological networks into highly connected clusters with maximum edge coverage.
IEEE/ACM Transactions on Computational Biology and Bioinformatics 11(3):455–467, 2014
(original publication).
[show abstract]
Abstract.
A popular clustering algorithm for biological networks which was
proposed by Hartuv and Shamir [IPL 2000] identifies
nonoverlapping highly connected components. We extend the
approach taken by this algorithm by introducing the
combinatorial optimization problem Highly
Connected Deletion, which asks for removing as few edges
as possible from a graph such that the resulting graph consists
of highly connected components. We show
that Highly Connected Deletion is
NPhard and provide a fixedparameter algorithm and a
kernelization. We propose exact and heuristic solution
strategies, based on polynomialtime data reduction rules and
integer linear programming with column generation. The data
reduction typically identifies 75 % of the edges that are deleted
for an optimal solution; the column generation method can then
optimally solve protein interaction networks with up to
6 000 vertices and 13 500 edges in less than a
day. Additionally, we present a new heuristic that finds more
clusters than the method by Hartuv and Shamir.

2013

Conference articles 

René van Bevern,
Robert Bredereck,
Morgan Chopin,
Sepp Hartung,
Falk Hüffner,
André Nichterlein, and
Ondřej Suchý:
Parameterized complexity of DAG partitioning.
In Proceedings of the 8th International Conference on Algorithms and Complexity
(CIAC ’13),
Barcelona, Spain. May 2013.
Volume 7878 in Lecture Notes in Computer Science, pages 49–60,
Springer, 2013 (original publication).
[show abstract]
Abstract.
The goal of tracking the origin of short, distinctive phrases
(memes) that propagate through the web in reaction to current events
has been formalized as DAG Partitioning: given a
directed acyclic graph, delete edges of minimum weight such that each
resulting connected component of the underlying undirected graph contains
only one sink. Motivated by NPhardness and hardness of approximation
results, we consider the parameterized complexity of this problem. We show
that it can be solved in O(2^{k} · n^{2}) time,
where k is the number of edge deletions, proving fixedparameter
tractability for parameter k. We then show that unless the Exponential
Time Hypothesis (ETH) fails, this cannot be improved to
2^{o(k)} · n^{O(1)};
further, DAG Partitioning does not have a polynomial
kernel unless NP ⊆ coNP/poly. Finally, given a tree decomposition of
width w, we show how to solve DAG Partitioning
in 2^{O(w2)} · n time, improving a
known algorithm for the parameter pathwidth.

Sharon Bruckner,
Falk Hüffner,
Christian Komusiewicz, and
Rolf Niedermeier:
Evaluation of ILPbased approaches for partitioning into colorful components.
In Proceedings of the 12th International Symposium on Experimental Algorithms
(SEA ’13),
Rome, Italy. June 2013.
Volume 7933 in Lecture Notes in Computer Science, pages 176–187,
Springer, 2013 (original publication).
[show abstract]
Abstract.
The NPhard Colorful Components problem
is a graph partitioning problem on vertexcolored graphs. We
identify a new application of Colorful
Components in the correction of Wikipedia interlanguage
links. We then describe and compare several exact and heuristic
approaches for solving Colorful
Components. In particular, we devise two ILP
formulations, where one employs a relationship to the
Hitting Set problem and the other one
uses a Clique Partition formulation.
Furthermore, we use the recently proposed implicit hitting set
framework [Karp, JCSS 2011, Chandrasekaran et al., SODA 2011] to
solve Colorful Components. Finally, we
also study a movebased and a mergebased heuristic for
Colorful Components. Our approach can
solve the Colorful Components model of
Wikipedia link correction optimally; while the
Clique Partitionbased ILP outperforms
the other two approaches, the implicit hitting set is a simple
approach that delivers competitive results. Finally, the
mergebased heuristic is very accurate and outperforms the
movebased one.

Falk Hüffner,
Christian Komusiewicz,
Adrian Liebtrau, and
Rolf Niedermeier:
Partitioning biological networks into highly connected clusters with maximum edge coverage.
In Proceedings of the 9th International Symposium on Bioinformatics Research and Applications
(ISBRA ’13),
Charlotte, North Carolina. May 2013.
Volume 7875 in Lecture Notes in Computer Science, pages 99–111,
Springer, 2013 (original publication).
Journal version available.
[show abstract]
Abstract.
We introduce the combinatorial optimization problem
Highly Connected Deletion, which
asks for removing as few edges as possible from a graph such
that the resulting graph consists of highly connected
components. We show that Highly Connected
Deletion is NPhard and provide a fixedparameter
algorithm and a kernelization. We propose exact and heuristic
solution strategies, based on polynomialtime data reduction
rules and integer linear programming with column generation. The
data reduction typically identifies 85% of the edges
that need to be deleted for an optimal solution; the column
generation method can then optimally solve protein interaction
networks with up to 5 000 vertices and
12 000 edges.

Journal articles 

Hartmut Ehrig,
Claudia Ermel,
Falk Hüffner,
Rolf Niedermeier, and
Olga Runge:
Confluence in data reduction: bridging graph transformation and kernelization.
Computability 2(1):31–49, 2013
(original publication).
[show abstract]
Abstract.
Kernelization is a core tool of parameterized algorithmics for
coping with computationally intractable problems. A
kernelization reduces in polynomial time an input
instance to an equivalent instance whose size is bounded by a
function only depending on some problemspecific
parameter k; this new instance is called problem
kernel. Typically, problem kernels are achieved by performing
efficient data reduction rules. So far, there was little
systematic study in the literature concerning the mutual
interaction of data reduction rules, in particular whether data
reduction rules for a specific problem always lead to the same
reduced instance, no matter in which order the rules are
applied. This corresponds to the concept of confluence from the
theory of rewriting systems. We argue that it is valuable to
study whether a kernelization is confluent, using the NPhard
graph problems (Edge) Clique Cover
and Partial Clique Cover as running
examples. We apply the concept of critical pair analysis from
graph transformation theory, supported by the AGG software tool.
These results support the main goal of our work, namely, to
establish a fruitful link between (parameterized) algorithmics
and graph transformation theory, two so far unrelated fields.

2012

Conference articles 

Sharon Bruckner,
Falk Hüffner,
Christian Komusiewicz,
Rolf Niedermeier,
Sven Thiel, and
Johannes Uhlmann:
Partitioning into colorful components by minimum edge deletions.
In Proceedings of the 23rd Annual Symposium on Combinatorial Pattern Matching
(CPM ’12),
Helsinki, Finland. July 2012.
Volume 7354 in Lecture Notes in Computer Science, pages 56–69,
Springer, 2012 (original publication).
[show abstract]
Abstract.
The NPhard Colorful Components problem is, given a
vertexcolored graph, to delete a minimum number of edges such that no
connected component contains two vertices of the same color. It has
applications in multiple sequence alignment and in multiple network alignment
where the colors correspond to species. We initiate a systematic
complexitytheoretic study of Colorful Components by
presenting NPhardness as well as fixedparameter tractability results for
different variants of Colorful Components. We also
perform experiments with our algorithms and additionally develop an efficient
and very accurate heuristic algorithm clearly outperforming a previous
mincutbased heuristic on multiple sequence alignment data.

Hartmut Ehrig,
Claudia Ermel,
Falk Hüffner,
Rolf Niedermeier, and
Olga Runge:
Confluence in data reduction: bridging graph transformation and kernelization.
In Proceedings of the 8th Conference on Computability in Europe
(CiE ’12),
Cambridge, UK. June 2012.
Volume 7318 in Lecture Notes in Computer Science, pages 193–202,
Springer, 2012 (original publication).
Journal version available.
[show abstract]
Abstract.
Kernelization is a core tool of parameterized algorithmics for coping with
computationally intractable problems. A kernelization reduces in polynomial time an
input instance to an equivalent instance whose size is bounded by a function only
depending on some problemspecific parameter k; this new instance is called
problem kernel. Typically, problem kernels are achieved by performing efficient data
reduction rules. So far, there was little study in the literature concerning the mutual
interaction of data reduction rules; in particular, it seems unstudied whether data
reduction rules for a specific problem always lead to the same reduced instance, no
matter in which order the rules are applied. This corresponds to the concept of
confluence from rewriting system theory. We argue that it is interesting to study
whether a kernelization is confluent. To this end, we use the NPhard graph
problem (Edge) Clique Cover as a running example, which asks
for the smallest set of cliques in a graph such that each edge is contained in at least
one clique. We provide a lineartime confluent kernelization for this problem and
demonstrate how to apply the concept of critical pair analysis from graph
transformation theory to achieve this, supported by the AGG software tool. We
generalize our investigations to the Partial Clique Cover
problem, which requires a more involved analysis. These results support the main goal
of our work, namely, to establish a link between (parameterized) algorithmics and graph
transformation theory, two so far unrelated fields whose interaction promises to be
beneficial for both sides.

2011

Conference articles 

Antonios Antoniadis,
Falk Hüffner,
Pascal Lenzner,
Carsten Moldenhauer, and
Alexander Souza:
Balanced interval coloring.
In Proceedings of the 28th Symposium on Theoretical Aspects of Computer Science
(STACS ’11),
Dortmund, Germany. March 2011.
Volume 9 in Leibniz International Proceedings in Informatics (LIPIcs), pages 531–542, Schloss Dagstuhl–LeibnizZentrum für Informatik
(original publication).
[show abstract]
Abstract.
We consider the discrepancy problem of coloring n
intervals with k colors such that at each point on the
line, the maximal difference between the number of intervals of
any two colors is minimal. Somewhat surprisingly, a coloring
with maximal difference at most one always exists. Furthermore,
we give an algorithm with running time O(n log
n + kn log k) for its construction. This is
in particular interesting because many known results for
discrepancy problems are nonconstructive. This problem
naturally models a load balancing scenario, where n tasks
with given start and endtimes have to be distributed among
k servers. Our results imply that this can be done
ideally balanced.
When generalizing to ddimensional boxes (instead of
intervals), a solution with difference at most one is not always
possible. We show that it is NPcomplete to decide this for any
d ≥ 2 and any k ≥ 2, which implies also
NPhardness of the respective minimization problem.
In an online scenario, where intervals arrive over time and the
color has to be decided upon arrival, the maximal difference in
the size of color classes can become arbitrarily high for any
online algorithm.

Britta Dorn,
Falk Hüffner,
Dominikus Krüger,
Rolf Niedermeier, and
Johannes Uhlmann:
Exploiting bounded signal flow for graph orientation based on cause–effect pairs.
In Proceedings of the 1st International ICST Conference on Theory and Practice of Algorithms
in (Computer) Systems
(TAPAS ’11),
Rome, Italy. April 2011.
Volume 6595 in Lecture Notes in Computer Science, pages 104–115, Springer
(original publication).
Journal version available.
[show abstract]
Abstract.
We consider the following problem: Given an undirected network
and a set of sender–receiver pairs, direct all edges
such that the maximum number of “signal flows”
defined by the pairs can be routed respecting edge
directions. This problem has applications in communication
networks and in understanding protein interaction based cell
regulation mechanisms. Since this problem is NPhard,
research so far concentrated on polynomialtime approximation
algorithms and tractable special cases. We take the viewpoint
of parameterized algorithmics and examine several parameters
related to the maximum signal flow over vertices or edges. We
provide several fixedparameter tractability results, and in
one case a sharp complexity dichotomy between a lineartime
solvable case and a slightly more general NPhard case. We
examine the value of these parameters for several realworld
network instances. For many relevant cases, the NPhard
problem can be solved to optimality. In this way,
parameterized analysis yields both deeper insight into the
computational complexity and practical solving strategies.

Journal articles 

Britta Dorn,
Falk Hüffner,
Dominikus Krüger,
Rolf Niedermeier, and
Johannes Uhlmann:
Exploiting bounded signal flow for graph orientation based on cause–effect pairs.
Algorithms for Molecular Biology 6(1):21, 2011
(original publication).
[show abstract]
Abstract.
Background: We consider the following problem: Given an
undirected network and a set of sender–receiver pairs, direct
all edges such that the maximum number of “signal flows” defined
by the pairs can be routed respecting edge directions. This
problem has applications in understanding protein interaction
based cell regulation mechanisms. Since this problem is
NPhard, research so far concentrated on polynomialtime
approximation algorithms and tractable special cases.
Results: We take the viewpoint of parameterized
algorithmics and examine several parameters related to the
maximum signal flow over vertices or edges. We provide several
fixedparameter tractability results, and in one case a sharp
complexity dichotomy between a lineartime solvable case and a
slightly more general NPhard case. We examine the value of
these parameters for several realworld network instances.
Conclusions: Several biologically relevant special cases
of the NPhard problem can be solved to optimality. In this way,
parameterized analysis yields both deeper insight into the
computational complexity and practical solving strategies.

Book chapters 

2010

Journal articles 

Sharon Bruckner,
Falk Hüffner,
Richard M. Karp,
Ron Shamir, and
Roded Sharan:
Topologyfree querying of protein interaction networks.
Journal of Computational Biology 17(3):237–252, 2010
(original publication).
[show abstract]
Abstract.
In the network querying problem, one is given a protein complex
or pathway of species A and a protein–protein
interaction network of species B; the goal is to
identify subnetworks of B that are similar to the
query in terms of sequence, topology, or both. Existing
approaches mostly depend on knowledge of the interaction
topology of the query in the network of species A;
however, in practice, this topology is often not known. To
address this problem, we develop a topologyfree querying
algorithm, which we call Torque. Given a
query, represented as a set of proteins, Torque seeks a matching set of proteins that
are sequencesimilar to the query proteins and span a connected
region of the network, while allowing both insertions and
deletions. The algorithm uses alternatively dynamic programming
and integer linear programming for the search task. We test
Torque with queries from yeast, fly, and
human, where we compare it to the QNet topologybased approach,
and with queries from less studied species, where only
topologyfree algorithms apply. Torque
detects many more matches than QNet, while giving results that
are highly functionally coherent.

Michael Dom,
Jiong Guo,
Falk Hüffner,
Rolf Niedermeier, and
Anke Truss:
Fixedparameter tractability results for feedback set problems in tournaments.
Journal of Discrete Algorithms 8(1):76–86, 2010
(original publication).
[show abstract]
Abstract.
Complementing recent progress on classical complexity and
polynomialtime approximability of feedback set problems in
(bipartite) tournaments, we extend and improve fixedparameter
tractability results for these problems. We show that Feedback Vertex Set in tournaments (FVST) is
amenable to the novel iterative compression technique, and we
provide a depthbounded search tree for Feedback Arc Set in bipartite tournaments
based on a new forbidden subgraph characterization. Moreover,
we apply the iterative compression technique to dHitting Set, which generalizes Feedback Vertex Set in tournaments, and obtain
improved upper bounds for the time needed to solve 4Hitting Set and 5Hitting
Set. Using our parameterized algorithm for Feedback Vertex Set in tournaments, we also
give an exact (not parameterized) algorithm for it running in
O(1.709^{n}) time, where n is the number
of input graph vertices, answering a question of Woeginger
[Discrete Appl. Math. 156(3):397–405, 2008].

Falk Hüffner,
Christian Komusiewicz,
Hannes Moser, and
Rolf Niedermeier:
Fixedparameter algorithms for cluster vertex deletion.
Theory of Computing Systems 47(1):196–217, 2010
(original publication).
[show abstract]
Abstract.
We initiate the first systematic study of the NPhard Cluster Vertex Deletion (CVD) problem (unweighted and
weighted) in terms of fixedparameter algorithmics. In the unweighted
case, one searches for a minimum number of vertex deletions to transform a
graph into a collection of disjoint cliques. The parameter is the number
of vertex deletions. We present efficient fixedparameter algorithms for
CVD applying the fairly new iterative compression technique. Moreover, we
study the variant of CVD where the maximum number of cliques to be
generated is prespecified. Here, we exploit connections to fixedparameter
algorithms for (weighted) Vertex Cover.

Falk Hüffner,
Nadja Betzler, and
Rolf Niedermeier:
Separatorbased data reduction for signed graph balancing.
Journal of Combinatorial Optimization 20(4):335–360, 2010
(original publication).
[show abstract]
Abstract.
Polynomialtime data reduction is a classical approach to hard
graph problems. Typically, particular small subgraphs are
replaced by smaller gadgets. We generalize this approach to
handle any small subgraph that has a small separator connecting
it to the rest of the graph. The problem we study is the NPhard
Balanced Subgraph problem, which asks
for a 2coloring of a graph that minimizes the inconsistencies
with given edge labels. It has applications in social networks,
systems biology, and integrated circuit design. The data
reduction scheme unifies and generalizes a number of previously
known data reductions, and can be applied to a large number of
graph problems where a coloring or a subset of the vertices is
sought. To solve the instances that remain after reduction, we
use a fixedparameter algorithm based on iterative compression
with a very effective heuristic speedup. Our implementation can
solve biological realworld instances exactly for which
previously only approximations were known. In addition, we
present experimental results for financial networks and random
networks.

Book chapters 

2009

Conference articles 

Sebastian Böcker,
Falk Hüffner,
Anke Truss, and
Magnus Wahlström:
A faster fixedparameter approach to drawing binary tanglegrams.
In Proceedings of the 4th International Workshop on Parameterized and Exact Computation
(IWPEC ’09),
Copenhagen, Denmark. September 2009.
Volume 5917 in Lecture Notes in Computer Science, pages 38–49, Springer
(original publication).
[show abstract]
Abstract.
Given two binary phylogenetic trees covering the same n
species, it is useful to compare them by drawing them with
leaves arranged sidebyside. To facilitate comparison, we would
like to arrange the trees to minimize the number of crossings
k induced by connecting pairs of identical species. This
is the NPhard Tanglegram Layout
problem. By providing a fast transformation to the Balanced Subgraph problem, we show that the
problem admits an O(2^{k}
n^{4}) algorithm, improving upon a
previous fixedparameter approach with running time
O(c^{k}
n^{O(1)}) where c ≈ 1000. We
enhance a Balanced Subgraph
implementation based on data reduction and iterative compression
with improvements tailored towards these instances, and run
experiments with realworld data to show the practical
applicability of this approach. All practically relevant
(k ≤ 1000) Tanglegram Layout
instances can be solved exactly within seconds. Additionally, we
provide a kernellike bound by showing how to reduce the Balanced Subgraph instances for Tanglegram Layout on complete binary trees to
a size of O(k log k).

Sharon Bruckner,
Falk Hüffner,
Richard M. Karp,
Ron Shamir, and
Roded Sharan:
Topologyfree querying of protein interaction networks.
In Proceedings of the 13th Annual International Conference on Research in Computational Molecular Biology
(RECOMB ’09),
Tucson, Arizona, USA. May 2009.
Volume 5541 in Lecture Notes in Bioinformatics, pages 74–89, Springer
(original publication).
Journal version available.
[show abstract]
Abstract.
In the network querying problem, one is given a protein complex
or pathway of species A and a proteinprotein
interaction network of species B; the goal is to
identify subnetworks of B that are similar to the
query. Existing approaches mostly depend on knowledge of the
interaction topology of the query in the network of
species A; however, in practice, this topology is
often not known. To combat this problem, we develop a
topologyfree querying algorithm, which we call Torque. Given a query, represented as a set of
proteins, Torque seeks a matching set of
proteins that are sequencesimilar to the query proteins and
span a connected region of the network, while allowing both
insertions and deletions. The algorithm uses alternatively
dynamic programming and integer linear programming for the
search task. We test Torque with queries
from yeast, fly, and human, where we compare it to the QNet
topologybased approach, and with queries from less studied
species, where only topologyfree algorithms apply. Torque detects many more matches than QNet,
while in both cases giving results that are highly functionally
coherent.

Journal articles 

Sharon Bruckner,
Falk Hüffner,
Richard M. Karp,
Ron Shamir, and
Roded Sharan:
Torque: Topologyfree querying of protein interaction networks.
Nucleic Acids Research 37(Web Server issue):W106–W108, 2009
(original publication).
[show abstract]
Abstract.
Torque is a tool for crossspecies
querying of protein–protein interaction networks. It aims to
answer the following question: given a set of proteins
constituting a known complex or a pathway in one species, can a
similar complex or pathway be found in the protein network of
another species? To this end, Torque
seeks a matching set of proteins that are sequencesimilar to
the query proteins and span a connected region of the target
network, while allowing for both insertions and
deletions. Unlike existing approaches, Torque does not require knowledge of the
interconnections among the query proteins. It can handle large
queries of up to 25 proteins.

Falk Hüffner:
Algorithm engineering for optimal graph bipartization.
Journal of Graph Algorithms and Applications, 13(2):77–98, 2009
(original publication).
[show abstract]
Abstract.
We examine exact algorithms for the NPhard Graph
Bipartization problem. The task is, given a graph, to find a
minimum set of vertices to delete to make it bipartite. Based on the
“iterative compression” method introduced by Reed, Smith, and
Vetta in 2004, we present new algorithms and experimental results.
The worstcase time complexity is improved. Based on new structural
insights, we give a simplified and more intuitive correctness proof. This
also allows to establish a heuristic improvement that in particular speeds
up the search on dense graphs. Our best algorithm can solve all instances
from a testbed from computational biology within minutes, whereas
established methods are only able to solve about half of the instances
within reasonable time.

Falk Hüffner:
Parametrisierte Ansätze für schwere Graphprobleme: Algorithmen und Experimente.
it – Information Technology, 51(3):171–174, 2009
(original publication).

Falk Hüffner,
Christian Komusiewicz,
Hannes Moser, and
Rolf Niedermeier:
Isolation concepts for clique enumeration: comparison and computational experiments.
Theoretical Computer Science, 410(52):5384–5397, 2009
(original publication).
[show abstract]
Abstract.
We do computational studies concerning the enumeration of
isolated cliques in graphs. Isolation, as recently introduced,
measures the degree of connectedness of the cliques to the rest
of the graph. Isolation helps both in getting faster algorithms
than for the enumeration of maximal general cliques and in
filtering out cliques with special semantics. We compare three
isolation concepts and their combination with two enumeration
modi for maximal cliques (“isolated maximal” vs
“maximal isolated”). All studied concepts exhibit
the fixedparameter tractability of the enumeration task with
respect to the parameter “degree of isolation”. We
provide a first systematic experimental study of the
corresponding enumeration algorithms, using synthetic graphs (in
the G_{n,m,p} model),
financial networks, and a music artist similarity network
(proposing the enumeration of isolated cliques as a useful
instrument in analyzing financial and social networks).

Christian Komusiewicz,
Falk Hüffner,
Hannes Moser, and
Rolf Niedermeier:
Isolation concepts for efficiently enumerating dense subgraphs.
Theoretical Computer Science, 410(38–40):3640–3654, 2009
(original publication).
[show abstract]
Abstract.
In an undirected graph G = (V, E), a set
of k vertices is called cisolated if it has less
than c · k outgoing edges. Ito and Iwama [ACM
Trans. Algorithms, to appear] gave an algorithm to enumerate all
cisolated maximal cliques in
O(4^{c} · c^{4} ·
E) time. We extend this to enumerating all maximal
cisolated cliques (which are a superset) and improve the
running time bound to O(2.89^{c} · c² ·
E), using modifications which also facilitate
parallelizing the enumeration. Moreover, we introduce a more
restricted and a more general isolation concept and show that
both lead to faster enumeration algorithms. Finally, we extend
our considerations to splexes (a relaxation of the
clique notion), providing a W[1]hardness result when the size
of the splex is the parameter and a fixedparameter
algorithm for enumerating isolated splexes when the
parameter describes the degree of isolation.

Book chapters 

Falk Hüffner,
Rolf Niedermeier, and
Sebastian Wernicke:
Fixedparameter algorithms for graphmodeled data clustering.
In:
Clustering Challenges in Biological Networks, chapter 1, pages 3–28,
World Scientific, 2009
(original publication).
[show abstract]
Abstract.
Fixedparameter algorithms can efficiently find optimal
solutions to some NPhard problems, including several problems
that arise in graphmodeled data clustering. This survey
provides a primer about practical techniques to develop such
algorithms; in particular, we discuss the design of
kernelizations (data reductions with provable
performance guarantees) and depthbounded search
trees. Our investigations are circumstantiated by three
concrete problems from the realm of graphmodeled data
clustering for which fixedparameter algorithms have been
implemented and experimentally evaluated, namely Clique, Cluster
Editing, and Clique Cover.

2008

Conference articles 

Jiong Guo,
Falk Hüffner,
Christian Komusiewicz, and
Yong Zhang:
Improved algorithms for bicluster editing.
In Proceedings of the 5th Annual Conference on Theory and Applications of Models
of Computation
(TAMC ’08),
Xian, China. April 2008.
Volume 4978 in Lecture Notes in Computer Science, pages 445–456, Springer
(original publication).
Note: After publication, we have been notified of a
problem in the proof of Theorem 4 (the factor4
approximation). Thus, we retract the results in Section 4. A
correct factor4 approximation algorithm was given by Nir Ailon,
Noa AvigdorElgrabli, Edo Liberty, and Anke van Zuylen:
Improved Approximation Algorithms for Bipartite Correlation Clustering.
[show abstract]
Abstract.
The NPhard Bicluster Editing is to add
or remove at most k edges to make a bipartite graph a
vertexdisjoint union of complete bipartite subgraphs. It has
applications in the analysis of gene expression data. We show
that by polynomialtime preprocessing, one can shrink a problem
instance to one with 4k vertices, thus proving that the
problem has a linear kernel, improving a quadratic kernel
result. We further give a search tree algorithm that improves
the running time bound from the trivial
O(4^{k} + m) to
O(3.24^{k} + m). Finally, we give a
randomized 4approximation, improving a known approximation with
factor 11.

Falk Hüffner,
Christian Komusiewicz,
Hannes Moser, and
Rolf Niedermeier:
Enumerating isolated cliques in synthetic and financial networks.
In Proceedings of the 2nd Annual International Conference on Combinatorial Optimization and Applications
(COCOA ’08),
St. John’s, Newfoundland, Canada. August 2008.
Volume 5165 in Lecture Notes in Computer Science, pages 405–416, Springer
(original publication).
Journal version available.
[show abstract]
Abstract.
We do computational studies concerning the enumeration of
maximal isolated cliques in graphs. Isolation, as recently
introduced, measures the degree of connectedness of the cliques
to the rest of the graph. Isolation helps both in getting faster
algorithms than for the enumeration of maximal general cliques
and in filtering out cliques with special semantics. Our
experiments cover synthetic graphs
(G_{n,m,p} model) and
financial networks; our results propose the enumeration of
isolated cliques as a useful instrument in analyzing financial
networks.

Falk Hüffner,
Christian Komusiewicz,
Hannes Moser, and
Rolf Niedermeier:
Fixedparameter algorithms for cluster vertex deletion.
In Proceedings of the 8th Latin American Theoretical Informatics Symposium
(LATIN ’08),
Búzios, Brazil. April 2008.
Volume 4957 in Lecture Notes in Computer Science, pages 711–722, Springer
(original publication).
Journal version available.
[show abstract]
Abstract.
We initiate the first systematic study of the NPhard Cluster Vertex Deletion (CVD) problem
(unweighted and weighted) in terms of fixedparameter
algorithmics. Here one searches for a minimum number of vertex
deletions to transform a graph into a collection of cliques. The
parameter denotes the number of vertex deletions. We present
several efficient fixedparameter algorithms for CVD. Our
iterative compression algorithm for CVD seems to be the first
nontrivial application of this fairly new technique to a problem
that is not a feedback set problem. Moreover, we study the
variant of CVD where the number of cliques to be generated is
prespecified. Here, we detect surprising connections to
fixedparameter algorithms for (Weighted)
Vertex Cover.

Oriana Ponta,
Falk Hüffner, and
Rolf Niedermeier:
Speeding up dynamic programming for some NPhard graph recoloring problems.
In Proceedings of the 5th Annual Conference on Theory and Applications of Models
of Computation
(TAMC ’08),
Xian, China. April 2008.
Volume 4978 in Lecture Notes in Computer Science, pages 490–501, Springer
(original publication).
[show abstract]
Abstract.
A vertex coloring of a tree is called convex if each color
induces a connected component. The Convex
Recoloring problem asks for a minimumweight change of colors
to achieve a convex coloring. For the nonuniformly weighted model,
where the cost of changing a vertex v to color c
depends on both v and c, we improve the running time
on trees from O(Δ^{κ} ·
κn) to O(3^{κ} ·
κn), where Δ is the maximum vertex degree
of the input graph G, κ is the number of colors,
and n is the number of vertices in G. In the uniformly
weighted case, where costs depend only on the vertex to be
recolored, one can instead parameterize on the number of
bad colors β ≤ κ, which is the
number of colors that do not already induce a connected
component. Here, we improve the running time from
O(Δ^{β} · βn) to
O(3^{β} · βn). The
improvements come from a more finegrained dynamic programming. For
the case where the weights are integers bounded by M, using
fast subset convolution, we further improve the running time with
respect to the exponential part to
O(2^{κ} ·
κ^{4}n^{2M}log^{2}(κnM))
and O(2^{β} ·
β^{4}n^{2M}
log^{2}(βnM)) for the nonuniformly weighted and
uniformly weighted model, respectively. Finally, we use fast subset
convolution to improve the exponential part of the running time of
the related 1Connected Coloring Completion
problem from O(8^{k} · k +
2^{k} · kn) to O(4^{k} ·
k^{4} log^{2} k + 2^{k}
· kn).

Journal articles 

Michael Dom,
Jiong Guo,
Falk Hüffner, and
Rolf Niedermeier:
Closest 4leaf power is fixedparameter tractable.
Discrete Applied Mathematics, 156(18):3345–3361, 2008
(original publication).
[show abstract]
Abstract.
The NPcomplete Closest 4Leaf Power
problem asks, given an undirected graph, whether it can be
modified by at most r edge insertions or deletions such
that it becomes a 4leaf power. Herein, a 4leaf power is a
graph that can be constructed by considering an unrooted
tree—the 4leaf root—with leaves onetoone labeled
by the graph vertices, where we connect two graph vertices by an
edge iff their corresponding leaves are at distance at most 4 in
the tree. Complementing previous work on Closest 2Leaf Power and Closest 3Leaf Power, we give the first
algorithmic result for Closest 4Leaf
Power, showing that Closest 4Leaf
Power is fixedparameter tractable with respect to the
parameter r.

Jens Gramm,
Jiong Guo,
Falk Hüffner, and
Rolf Niedermeier:
Data reduction and exact algorithms for clique cover.
ACM Journal of Experimental Algorithmics, 13, Article 2.2, 15 pages, 2008
(original publication).
[show abstract]
Abstract.
To cover the edges of a graph with a minimum number of cliques
is an NPhard problem with many applications. We develop for
this problem efficient and effective polynomialtime data
reduction rules that, combined with a search tree algorithm,
allow for exact problem solutions in competitive time. This is
confirmed by experiments with realworld and synthetic
data. Moreover, we prove the fixedparameter tractability of
covering edges by cliques.

Jiong Guo,
Falk Hüffner,
Erhan Kenar,
Rolf Niedermeier, and
Johannes Uhlmann:
Complexity and exact algorithms for vertex multicut in interval
and bounded treewidth graphs.
European Journal of Operational Research,
186(2):542–553, 2008
(original publication).
[show abstract]
Abstract.
Multicut is a fundamental network
communication and connectivity problem. It is defined as: given
an undirected graph and a collection of pairs of terminal
vertices, find a minimum set of edges or vertices whose removal
disconnects each pair. We mainly focus on the case of removing
vertices, where we distinguish between allowing or disallowing the
removal of terminal vertices. Complementing and refining previous
results from the literature, we provide several NPcompleteness
and (fixedparameter) tractability results for restricted classes
of graphs such as trees, interval graphs, and graphs of bounded
treewidth.

Falk Hüffner,
Rolf Niedermeier, and
Sebastian Wernicke:
Techniques for Practical FixedParameter Algorithms.
The Computer Journal,
51(1):7–25, 2008
(original publication).
[show abstract]
Abstract.
The fixedparameter approach is an algorithm design technique
for solving combinatorially hard (mostly NPhard) problems. For
some of these problems, it can lead to algorithms that are both
efficient and yet at the same time guaranteed to find optimal
solutions. Focusing on their application to solving NPhard
problems in practice, we survey three main techniques to develop
fixedparameter algorithms, namely kernelisation (data reduction
with provable performance guarantee), depthbounded search
trees, and a new technique called iterative compression. Our
discussion is circumstantiated by several concrete case studies
and provides pointers to various current challenges in the
field.

Falk Hüffner,
Sebastian Wernicke, and
Thomas Zichner:
Algorithm engineering for colorcoding with applications to signaling pathway detection.
Algorithmica,
52(2):114–132, 2008
(original publication).
[show abstract]
Abstract.
Colorcoding is a technique to design fixedparameter algorithms for several
NPcomplete subgraph isomorphism problems. Somewhat surprisingly, not much work has
so far been spent on the actual implementation of algorithms that are based on
colorcoding, despite the elegance of this technique and its wide range of
applicability to practically important problems. This work gives various novel
algorithmic improvements for colorcoding, both from a worstcase perspective as
well as under practical considerations. We apply the resulting implementation to the
identification of signaling pathways in protein interaction networks, demonstrating
that our improvements speed up the colorcoding algorithm by orders of magnitude
over previous implementations. This allows more complex and larger structures to be
identified in reasonable time; many biologically relevant instances can even be
solved in seconds where, previously, hours were required.

Book chapters 

2007

Conference articles 

Falk Hüffner,
Nadja Betzler, and
Rolf Niedermeier:
Optimal edge deletions for signed graph balancing.
In Proceedings of the 6th Workshop on Experimental Algorithms
(WEA ’07),
Rome, Italy. June 2007.
Volume 4525 in Lecture Notes in Computer Science, pages 297–310, Springer
(original publication).
Journal version available.
[show abstract]
Abstract.
The Balanced Subgraph problem (edge
deletion variant) asks for a 2coloring of a graph that
minimizes the inconsistencies with given edge labels. It has
applications in social networks, systems biology, and integrated
circuit design. We present an exact algorithm for Balanced Subgraph based on a combination of
data reduction rules and a fixedparameter algorithm. The data
reduction is based on finding small separators and a novel
gadget construction scheme. The fixedparameter algorithm is
based on iterative compression with a very effective heuristic
speedup. Our implementation can solve biological realworld
instances exactly for which previously only approximations
[DasGupta et al., WEA 2006] were known.

Falk Hüffner,
Sebastian Wernicke, and
Thomas Zichner:
Algorithm engineering for colorcoding to facilitate signaling pathway detection.
In Proceedings of the 5th AsiaPacific Bioinformatics Conference
(APBC ’07),
Hong Kong. January 2007.
Volume 5 in Advances in Bioinformatics and Computational Biology, pages 277–286,
Imperial College Press
(original publication).
Journal version available.
[show abstract]
Abstract.
Colorcoding is a technique to design fixedparameter algorithms
for several NPcomplete subgraphisomorphism problems. Somewhat
surprisingly, not much work has so far been spent on the actual
implementation of algorithms that are based on colorcoding,
despite the elegance of this technique and its wide range of
applicability to practically important problems. This work gives
various novel algorithmic improvements for colorcoding, both
from a worstcase perspective as well as under practical
considerations. We apply the resulting implementation to the
identification of signaling pathways in protein interaction
networks, demonstrating that our improvements speed up the
colorcoding algorithm by orders of magnitude over previous
implementations. This allows more complex and larger structures
to be identified in reasonable time; many biologically relevant
instances can even be solved in seconds where, previously, hours
were required.

Christian Komusiewicz,
Falk Hüffner,
Hannes Moser, and
Rolf Niedermeier:
Isolation concepts for enumerating dense subgraphs.
In Proceedings of the 13th International Computing and Combinatorics Conference
(COCOON ’07),
Banff, Canada. July 2007.
Volume 4598 in Lecture Notes in Computer Science, pages 140–150, Springer
(original publication).
Journal version available.
[show abstract]
Abstract.
In a graph G = (V,E), a vertex subset
S⊆;V of size k is called
cisolated if it has less than c·k outgoing
edges. We repair a nontrivially flawed algorithm for
enumerating all cisolated cliques due to Ito et al. [ESA
2005] and obtain an algorithm running in
O(4^{c}·c^{4}·E)
time. We describe a speedup trick that also helps parallelizing
the enumeration. Moreover, we introduce a more restricted and a
more general isolation concept and show that both lead to faster
enumeration algorithms. Finally, we extend our considerations
to splexes (a relaxation of the clique notion),
providing a W[1]hardness result and a fixedparameter algorithm
for enumerating isolated splexes.

Journal articles 

Jens Gramm,
Jiong Guo,
Falk Hüffner,
Rolf Niedermeier,
HansPeter Piepho, and
Ramona Schmid:
Algorithms for compact letter displays: comparison and evaluation.
Computational Statistics & Data Analysis,
52(2):725–736, 2007
(original publication).
[show abstract]
Abstract.
Multiple pairwise comparisons are one of the most frequent tasks
in applied statistics. In this context, letters
displays may be used for a compact presentation of results
of multiple comparisons. A heuristic previously proposed for
this task is compared with two new algorithmic approaches. The
latter rely on the equivalence of computing compact letters
displays to computing clique covers in graphs, a
problem that is wellstudied in theoretical computer science. A
thorough discussion of the three approaches aims to give a
comparison of the algorithms’ advantages and disadvantages. The
three algorithms are compared in a series of experiments on
simulated and real data, e.g., using data from wheat and
triticale yield trials.

Jiong Guo,
Falk Hüffner, and
Hannes Moser:
Feedback arc set in bipartite tournaments is NPcomplete.
Information Processing Letters,
102(2–3): 62–65, 2007
(original publication).
[show abstract]
Abstract.
The Feedback Arc Set problem asks whether it
is possible to delete at most k arcs to make a directed graph
acyclic. We show that Feedback Arc Set is
NPcomplete for bipartite tournaments, that is, directed graphs
that are orientations of complete bipartite graphs.

Falk Hüffner,
Sebastian Wernicke, and
Thomas Zichner:
FASPAD: fast signaling pathway detection.
Bioinformatics
Applications Note,
23(13): 1708–1709, 2007
(original publication).
[show abstract]
Abstract.
Faspad is a userfriendly tool that
detects candidates for linear signaling pathways in protein
interaction networks based on an approach by Scott et
al. [Journal of Computational Biology, 2006]. Using recent
algorithmic insights, it can solve the underlying NPhard
problem quite fast: For protein networks of typical size
(several thousand nodes), pathway candidates of length up to 13
proteins can be found within seconds and with a 99.9%
probability of optimality. Faspad
graphically displays all candidates that are found; for
evaluation and comparison purposes, an overlay of several
candidates and the surrounding network context can also be
shown.
 FASPAD for Linux (i386) or Windows (i386) and
as source code

Theses 

Falk Hüffner:
Algorithms and Experiments for Parameterized Approaches to Hard Graph Problems.
PhD thesis, Institut für Informatik, FriedrichSchillerUniversität Jena, 2007.
[show abstract]
Abstract.
This thesis is about the design, analysis, implementation, and
experimental evaluation of algorithms for hard graph problems. The
aim is to establish that the concept of fixedparameter
tractability, and in particular novel algorithmic techniques whose
development was driven by this concept, can lead to practically
useful programs for exactly solving realworld problem instances.
In particular, we examine graph problems such as Clique Cover, Balanced
Subgraph, and MinimumWeight Path,
which have applications in computational biology and other areas.
We mainly use the techniques of data reduction, iterative
compression, and colorcoding. We develop a number of new
fixedparameter algorithms and give implementations for four
important problems, all of which are to the best of our knowledge
the currently fastest solvers.

2006

Conference articles 

Matthias Brosemann,
Jochen Alber,
Falk Hüffner, and
Rolf Niedermeier:
Matrix robustness, with an application to power system observability.
In Proceedings of the 2nd Algorithms and Complexity in Durham
(ACiD ’06),
Durham, England. September 2006.
Volume 7 in Texts in Algorithmics, pages 37–48, College Publications.
[show abstract]
Abstract.
We initiate the study of the computational complexity of
Matrix Robustness: Check whether deleting any k rows from a
fullrank matrix does not change the matrix rank. This problem is
motivated by applications in observability analysis of electrical power
networks. We indicate the coNPcompleteness of Matrix
Robustness, provide linear programming based solutions (both exact
and heuristic) and a problemspecific algorithm, and present encouraging
experimental results.

Michael Dom,
Jiong Guo,
Falk Hüffner,
Rolf Niedermeier, and
Anke Truß:
Fixedparameter tractability results for feedback set problems in tournaments.
In Proceedings of the 6th Conference on Algorithms and Complexity
(CIAC ’06),
Rome, Italy. May 2006.
Volume 3998 in Lecture Notes in Computer Science, pages 320–331, Springer
(original publication).
Journal version available.
[show abstract]
Abstract.
Complementing recent progress on classical complexity and polynomialtime
approximability of feedback set problems in (bipartite) tournaments, we
extend and partially improve fixedparameter tractability results for
these problems. We show that Feedback Vertex Set
in tournaments is amenable to the novel iterative compression
technique. Moreover, we provide data reductions and problem kernels for
Feedback Vertex Set and Feedback
Arc Set in tournaments, and a depthbounded search tree for Feedback Arc Set in bipartite tournaments based on a new
forbidden subgraph characterization.

Jens Gramm,
Jiong Guo,
Falk Hüffner, and
Rolf Niedermeier:
Data reduction, exact, and heuristic algorithms for clique cover.
In Proceedings of the 8th Workshop on Algorithm Engineering and Experiments
(ALENEX ’06),
Miami, USA. January 2006.
Pages 86–94, SIAM
(original publication).
Journal version available.
[show abstract]
Abstract.
To cover the edges of a graph with a minimum number of cliques is an
NPcomplete problem with many applications. The stateoftheart solving
algorithm is a polynomialtime heuristic from the 1970s. We present an
improvement of this heuristic. Our main contribution, however, is the
development of efficient and effective polynomialtime data reduction
rules that, combined with a search tree algorithm, allow for exact problem
solutions in competitive time. This is confirmed by experiments with
realworld and synthetic data. Moreover, we prove the fixedparameter
tractability of covering edges by cliques.

Jiong Guo,
Falk Hüffner,
Erhan Kenar,
Rolf Niedermeier, and
Johannes Uhlmann:
Complexity and exact algorithms for multicut.
In Proceedings of the 32nd International Conference on Current Trends in Theory and Practice of Computer Science
(SOFSEM ’06),
Merin, Czech Republic. January 2006.
Volume 3831 in Lecture Notes in Computer Science, pages 303–312, Springer
(original publication).
Journal version available.

Journal articles 

Michael Dom,
Jiong Guo,
Falk Hüffner, and
Rolf Niedermeier:
Error compensation in leaf power problems.
Algorithmica,
44(4): 363–381, 2006
(original publication).
[show abstract]
Abstract.
The kLeaf Root recognition problem is a
particular case of graph power problems: For a given graph it asks whether
there exists an unrooted tree—the kleaf root—with
leaves onetoone labeled by the graph vertices and where the leaves have
distance at mostk iff their corresponding vertices in the graph are
connected by an edge. Here we study “error correction”
versions of Leaf Root recognition—that is,
adding or deleting at most l edges to generate a graph that has a
kleaf root. We provide several NPcompleteness results in this
context, and we show that the NPcomplete Closest 3Leaf
Root problem (the error correction version of 3Leaf Root) is fixedparameter tractable with respect
to the number of edge modifications or vertex deletions in the given
graph. Thus, we provide the seemingly first nontrivial positive
algorithmic results in the field of error compensation for leaf power
problems with k > 2. To this end, as a result of independent
interest, we develop a forbidden subgraph characterization of graphs with
3leaf roots.

Jiong Guo,
Jens Gramm,
Falk Hüffner,
Rolf Niedermeier, and
Sebastian Wernicke:
Compressionbased fixedparameter algorithms for Feedback Vertex Set and Edge Bipartization.
Journal of Computer and System Sciences,
72(8): 1386–1396, 2006
(original publication).
[show abstract]
Abstract.
Settling a ten years open question, we show that the NPcomplete Feedback Vertex Set problem is deterministically
solvable in O(c^{k} · m) time, where m
denotes the number of graph edges, k denotes the size of the
feedback vertex set searched for, and c is a constant. As a second
result, we present a fixedparameter algorithm for the NPcomplete Edge Bipartization problem with runtime
O(2^{k} · m^{2}).

2005

Conference articles 
 Michael Dom,
Jiong Guo,
Falk Hüffner, and
Rolf Niedermeier:
Extending the tractability border for closest leaf powers.
In Proceedings of the 31st International Workshop on
GraphTheoretic Concepts in Computer Science
(WG ’05),
Metz, France. June 2005.
Volume 3787 in Lecture Notes in Computer Science, pages 397–408, Springer
(original publication).
Journal version available.
[show abstract]
Abstract.
The NPcomplete Closest 4Leaf Power problem asks,
given an undirected graph, whether it can be modified by at most l
edge insertions or deletions such that it becomes a 4leaf power. Herein,
a 4leaf power is a graph that can be constructed by considering an
unrooted tree—the 4leaf root—with leaves onetoone labeled
by the graph vertices, where we connect two graph vertices by an edge iff
their corresponding leaves are at distance at most 4 in the tree.
Complementing and “completing” previous work on Closest 2Leaf Power and Closest 3Leaf
Power, we show that Closest 4Leaf Power is
fixedparameter tractable with respect to parameter l. This gives
one of the first and so far deepest positive algorithmic results in the
field of “approximate graph power recognition”—note that
for 5leaf powers even the complexity of the exact recognition problem is
open.
 Jiong Guo,
Jens Gramm,
Falk Hüffner,
Rolf Niedermeier, and
Sebastian Wernicke:
Improved fixedparameter algorithms for two feedback set problems.
In Proceedings of the 9th Workshop on Algorithms and Data Structures
(WADS ’05),
Waterloo, Canada. August 2005.
Volume 3608 in Lecture Notes in Computer Science, pages 158–168, Springer
(original publication).
Journal version available.
 Falk Hüffner:
Algorithm engineering for optimal graph bipartization.
In Proceedings of the 4th International Workshop on Efficient and
Experimental Algorithms
(WEA ’05),
Santorini Island, Greece. May 2005.
Volume 3503 in Lecture Notes in Computer Science, pages 240–252, Springer
(original publication).
Journal version available.
[show abstract]
Abstract.
We examine exact algorithms for the NPcomplete Graph
Bipartization problem that asks for a minimum set of vertices to
delete from a graph to make it bipartite. Based on the “iterative
compression” method recently introduced by Reed, Smith, and Vetta,
we present algorithmic improvements and experimental results. The
worstcase time complexity is improved from O(3^{k} ·
kmn) to O(3^{k} · mn), where n is the
number of vertices, m is the number of edges, and k is the
number of vertices to delete. Our best algorithm can solve all problems
from a testbed from computational biology within minutes, whereas
established methods are only able to solve about half of the problems
within reasonable time.

Journal articles 

Jens Gramm,
Jiong Guo,
Falk Hüffner, and
Rolf Niedermeier:
Graphmodeled data clustering: Exact algorithms for clique generation.
Theory of Computing Systems,
38(4):373–392, 2005
(original publication).
[show abstract]
Abstract.
We present efficient fixedparameter algorithms for the NPcomplete edge
modification problems Cluster Editing and Cluster Deletion. Here, the goal is to make the fewest
changes to the edge set of an input graph such that the new graph is a
vertexdisjoint union of cliques. Allowing up to k edge additions
and deletions (Cluster Editing), we solve this
problem in O(2.27^{k} + V^{3})
time; allowing only up to k edge deletions (Cluster Deletion), we solve this problem in
O(1.77^{k} + V^{3}) time. The key
ingredients of our algorithms are two easy to implement bounded search
tree algorithms and a reduction to a problem kernel of size
O(k^{3}). This improves and complements previous
work. Finally, we discuss further improvements on search tree sizes using
computergenerated case distinctions.

2004

Conference articles 

Michael Dom,
Jiong Guo,
Falk Hüffner, and
Rolf Niedermeier:
Error compensation in leaf root problems.
In Proceedings of the 15th Annual International Symposium on
Algorithms and Computation
(ISAAC ’04),
Hong Kong, China. December 2004.
Volume 3341 in Lecture Notes in Computer Science, pages 389–401, Springer
(original publication).
Journal version available.
 Jiong Guo,
Falk Hüffner, and
Rolf Niedermeier:
A structural view on parameterizing problems: distance from triviality.
In Proceedings of the International Workshop on Parameterized and Exact Computation
(IWPEC ’04),
Bergen, Norway. September 2004.
Volume 3162 in Lecture Notes in Computer Science, pages 162–173, Springer
(original publication).
[show abstract]
Abstract.
Based on a series of known and new examples, we propose the generalized
setting of “distance from triviality” measurement as a
reasonable and prospective way of determining useful structural problem
parameters in analyzing computationally hard problems. The underlying idea
is to consider tractable special cases of generally hard problems and to
introduce parameters that measure the distance from these special
cases. In this paper we present several case studies of distance from
triviality parameterizations (concerning Clique,
Power Dominating Set, Set
Cover, and Longest Common Subsequence) that
exhibit the versatility of this approach to develop important new views
for computational complexity analysis.

Journal articles 

Jens Gramm,
Jiong Guo,
Falk Hüffner, and
Rolf Niedermeier:
Automated generation of search tree algorithms for hard graph
modification problems.
Algorithmica,
39(4):321–347, 2004
(original publication).
[show abstract]
Abstract.
We present a framework for an automated generation of exact search tree
algorithms for NPhard problems. The purpose of our approach is
twofold—rapid development and improved upper bounds. Many search
tree algorithms for various problems in the literature are based on
complicated case distinctions. Our approach may lead to a much simpler
process of developing and analyzing these algorithms. Moreover, using the
sheer computing power of machines it may also lead to improved upper
bounds on search tree sizes (i.e., faster exact solving algorithms) in
comparison with previously developed “handmade” search trees.
Among others, such an example is given with the NPcomplete Cluster Editing problem (also known as Correlation Clustering on complete unweighted graphs),
which asks for the minimum number of edge additions and deletions to
create a graph which is a disjoint union of cliques. The handmade search
tree for Cluster Editing had worstcase size
O(2.27^{k}), which now is improved to
O(1.92^{k}) due to our new method. (Herein,
k denotes the number of edge modifications allowed.)

2003

Conference articles 

Jens Gramm,
Jiong Guo,
Falk Hüffner, and
Rolf Niedermeier:
Graphmodeled data clustering: fixedparameter algorithms for clique generation.
In Proceedings of the 5th Italian Conference on Algorithms and Complexity
(CIAC ’03),
Rome, Italy. May 2003.
Volume 2653 in Lecture Notes in Computer Science, pages 108–119, Springer
(original publication).
Journal version available.

Jens Gramm,
Jiong Guo,
Falk Hüffner, and
Rolf Niedermeier:
Automated generation of search tree algorithms for graph modification problems.
In Proceedings of the 11th Annual European Symposium on Algorithms
(ESA ’03),
Budapest, Hungary. September 2003.
Volume 2832 in Lecture Notes in Computer Science, pages 642–653, Springer
(original publication).
Journal version available.

Theses 

Falk Hüffner:
Graph Modification Problems and Automated Search Tree Generation.
Diplomarbeit, WilhelmSchickardInstitut für Informatik,
Universität Tübingen, October 2003.
[show abstract]
Abstract.
We present a framework for the automated generation of exact searchtree
algorithms for NPhard problems. The purpose of our approach is twofold:
rapid development and improved upper bounds.
Many search tree algorithms for various problems in the literature are
based on complicated case distinctions. Our approach may lead to a much
simpler process of developing and analyzing these algorithms. Moreover,
using the sheer computing power of machines it can also provide improved
upper bounds on search tree sizes (i.e., faster exact solving algorithms)
in comparison with previously developed “handmade” search
trees. The generated search tree algorithms work by expanding a
“window view” of the problem with local information, and then
branch according to a precalculated rule for the resulting window.
A central example in this work is given with the NPcomplete Cluster Editing problem, which asks for the minimum
number of edge additions and deletions in a graph to create a cluster
graph (i.e., a graph which is a disjoint union of cliques). For Cluster Editing, a handmade search tree is known with
O(2.27^{k}) worstcase size, which is improved to
O(1.92^{k}) due to our new method. (Herein,
k denotes the number of edge modifications allowed.) A refinement
of the search tree generator is presented, which tries to direct the
expansion of the window towards advantageous case sets. The scheme is also
successfully applied to several other graph modifications problems;
generalizations to other problems are sketched.

2002

Posters 

Theses 

Falk Hüffner:
Finding Optimal Solutions to Atomix.
Studienarbeit, WilhelmSchickardInstitut für Informatik,
University of Tübingen, December 2002.
[show abstract]
Abstract.
We present first solutions of benchmark instances to the solitaire
computer game Atomix found with different heuristic search methods. The
problem is shown to be NPhard. An implementation of the heuristic
algorithm A* is presented that needs no priority queue, thereby having
very low memory overhead. The limited memory algorithm IDA* is handicapped
by the fact that, due to move transpositions, duplicates appear very
frequently in the problem space; several schemes of using memory to
mitigate this weakness are explored, among those “partial”
schemes which trade memory savings for a small probability of not finding
an optimal solution. Even though the underlying search graph is directed,
backward search is shown to be viable, since the branching factor is the
same as for forward search.

2001

Conference articles 
