### Harnessing the Multicores: Nested Data Parallelism in Haskell

Simon Peyton Jones, Roman Leshchinskiy, Gabriele Keller & Manuel M T Chakravarty
If you want to program a parallel computer, a purely functional language like Haskell is a promising starting point. Since the language is pure, it is by-default safe for parallel evaluation, whereas imperative languages are by-default unsafe. But that doesn\'t make it easy! Indeed it has proved quite difficult to get robust, scalable performance increases through parallel functional programming, especially as the number of processors increases. A particularly promising and well-studied approach to employing large...

### Kernel(s) for Problems with No Kernel: On Out-Trees with Many Leaves

Henning Fernau, Fedor V. Fomin, Daniel Lokshtanov, Daniel Raible, Saket Saurabh & Yngve Villanger
The {\sc $k$-Leaf Out-Branching} problem is to find an out-branching, that is a rooted oriented spanning tree, with at least $k$ leaves in a given digraph. The problem has recently received much attention from the viewpoint of parameterized algorithms. Here, we take a kernelization based approach to the {\sc $k$-Leaf-Out-Branching} problem. We give the first polynomial kernel for {\sc Rooted $k$-Leaf-Out-Branching}, a variant of {\sc $k$-Leaf-Out-Branching} where the root of the tree searched for is...

### On Estimation Algorithms vs Approximation Algorithms

Uriel Feige
In a combinatorial optimization problem, when given an input instance, one seeks a feasible solution that optimizes the value of the objective function. Many combinatorial optimization problems are NP-hard. A way of coping with NP-hardness is by considering approximation algorithms. These algorithms run in polynomial time, and their performance is measured by their approximation ratio: the worst case ratio between the value of the solution produced and the value of the (unknown) optimal solution. In...

### 2008 Preface -- IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science

Ramesh Hariharan, Madhavan Mukund & V Vinay
This volume contains the proceedings of the 28th international conference on the Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2008), organized under the auspices of the Indian Association for Research in Computing Science (IARCS). This year's conference attracted 117 submissions. Each submission was reviewed by at least three independent referees. The final selection of the papers making up the programme was done through an electronic discussion on EasyChair, spanning two weeks, without a...

### A Stronger LP Bound for Formula Size Lower Bounds via Clique Constraints

Kenya Ueno
We introduce a new technique proving formula size lower bounds based on the linear programming bound originally introduced by Karchmer, Kushilevitz and Nisan (1995) and the theory of stable set polytope. We apply it to majority functions and prove their formula size lower bounds improved from the classical result of Khrapchenko (1971). Moreover, we introduce a notion of unbalanced recursive ternary majority functions motivated by a decomposition theory of monotone self-dual functions and give integrally...

### Preface -- 26th International Symposium on Theoretical Aspects of Computer Science

Susanne Albers & Jean-Yves Marion
The interest in STACS has remained at a high level over the past years. The STACS 2009 call for papers led to over 280 submissions from 41 countries. Each paper was assigned to three program committee members. The program committee held a two-week electronic meeting at the beginning of November and selected 54 papers. As co-chairs of the program committee, we would like to sincerely thank its members and the many external referees for their...

### On the Power of Imperfect Information

Dietmar Berwanger & Laurent Doyen
We present a polynomial-time reduction from parity games with imperfect information to safety games with imperfect information. Similar reductions for games with perfect information typically increase the game size exponentially. Our construction avoids such a blow-up by using imperfect information to realise succinct counters which cover a range exponentially larger than their size. In particular, the reduction shows that the problem of solving imperfect-information games with safety conditions is \EXPTIME-complete.

### Nonclairvoyant Speed Scaling for Flow and Energy

Ho-Leung Chan, Jeff Edmonds, Tak-Wah Lam, Lap-Kei Lee, Alberto Marchetti-Spaccamela & Kirk Pruhs
We study online nonclairvoyant speed scaling to minimize total flow time plus energy. We first consider the traditional model where the power function is $P(s)=s^\alpha$. We give a nonclairvoyant algorithm that is shown to be $O(\alpha^3)$-competitive. We then show an $\Omega( \alpha^{1/3-\epsilon} )$ lower bound on the competitive ratio of any nonclairvoyant algorithm. We also show that there are power functions for which no nonclairvoyant algorithm can be $O(1)$-competitive.

### Hardness and Algorithms for Rainbow Connectivity

Sourav Chakraborty, Eldar Fischer, Arie Matsliah & Raphael Yuster
An edge-colored graph $G$ is {\em rainbow connected} if any two vertices are connected by a path whose edges have distinct colors. The {\em rainbow connectivity} of a connected graph $G$, denoted $rc(G)$, is the smallest number of colors that are needed in order to make $G$ rainbow connected. In addition to being a natural combinatorial problem, the rainbow connectivity problem is motivated by applications in cellular networks. In this paper we give the first...

### About models of security protocols

Hubert Comon-Lundh
In this paper, mostly consisting of definitions, we revisit the models of security protocols: we show that the symbolic and the computational models (as well as others) are instances of a same generic model. Our definitions are also parametrized by the security primitives, the notion of attacker and, to some extent, the process calculus.

### Bounded Size Graph Clustering with Applications to Stream Processing

Rohit Khandekar, Kirsten Hildrum, Sujay Parekh, Deepak Rajan, Jay Sethuraman & Joel Wolf
We introduce a graph clustering problem motivated by a stream processing application. Input to our problem is an undirected graph with vertex and edge weights. A cluster is a subset of the vertices. The {\em size} of a cluster is defined as the total vertex weight in the subset plus the total edge weight at the boundary of the cluster. The bounded size graph clustering problem ($\GC$) is to partition the vertices into clusters of...

### Undecidable Properties of Limit Set Dynamics of Cellular Automata

Pietro Di Lena & Luciano Margara
Cellular Automata (CA) are discrete dynamical systems and an abstract model of parallel computation. The limit set of a cellular automaton is its maximal topological attractor. A well know result, due to Kari, says that all nontrivial properties of limit sets are undecidable. In this paper we consider properties of limit set dynamics, i.e. properties of the dynamics of Cellular Automata restricted to their limit sets. There can be no equivalent of Kari's Theorem for...

### Ambiguity and Communication

Juraj Hromkovic & Georg Schnitger
The ambiguity of a nondeterministic finite automaton (NFA) $N$ for input size $n$ is the maximal number of accepting computations of $N$ for an input of size $n$. For all $k,r \in \mathbb{N}$ we construct languages $L_{r,k}$ which can be recognized by NFA's with size $k \cdot$poly$(r)$ and ambiguity $O(n^k)$, but $L_{r,k}$ has only NFA's with exponential size, if ambiguity $o(n^k)$ is required. In particular, a hierarchy for polynomial ambiguity is obtained, solving a long...

### On the Tightening of the Standard SDP for Vertex Cover with $ell_1$ Inequalities

Konstantinos Georgiou, Avner Magen & Iannis Tourlakis
We show that the integrality gap of the standard SDP for \vc~on instances of $n$ vertices remains $2-o(1)$ even after the addition of \emph{all} hypermetric inequalities. Our lower bound requires new insights into the structure of SDP solutions behaving like $\ell_1$ metric spaces when one point is removed. We also show that the addition of all $\ell_1$ inequalities eliminates any solutions that are not convex combination of integral solutions. Consequently, we provide the strongest possible...

### A Unified Algorithm for Accelerating Edit-Distance Computation via Text-Compression

Danny Hermelin, Gad M. Landau, Shir Landau & Oren Weimann
The edit distance problem is a classical fundamental problem in computer science in general, and in combinatorial pattern matching in particular. The standard dynamic-programming solution for this problem computes the edit-distance between a pair of strings of total length $O(N)$ in $O(N^2)$ time. To this date, this quadratic upper-bound has never been substantially improved for general strings. However, there are known techniques for breaking this bound in case the strings are known to compress well...

### Weak MSO with the Unbounding Quantifier

Mikolaj Bojanczyk
A new class of languages of infinite words is introduced, called the \emph{max-regular languages}, extending the class of $\omega$-regular languages. The class has two equivalent descriptions: in terms of automata (a type of deterministic counter automaton), and in terms of logic (weak monadic second-order logic with a bounding quantifier). Effective translations between the logic and automata are given.

### Extracting the Kolmogorov Complexity of Strings and Sequences from Sources with Limited Independence

Marius Zimand
An infinite binary sequence has randomness rate at least $\sigma$ if, for almost every $n$, the Kolmogorov complexity of its prefix of length $n$ is at least $\sigma n$. It is known that for every rational $\sigma \in (0,1)$, on one hand, there exists sequences with randomness rate $\sigma$ that can not be effectively transformed into a sequence with randomness rate higher than $\sigma$ and, on the other hand, any two independent sequences with randomness...

### A Complexity Dichotomy for Partition Functions with Mixed Signs

Leslie Ann Goldberg, Martin Grohe, Mark Jerrum & Marc Thurley
\emph{Partition functions}, also known as \emph{homomorphism functions}, form a rich family of graph invariants that contain combinatorial invariants such as the number of $k$-colourings or the number of independent sets of a graph and also the partition functions of certain spin glass'' models of statistical physics such as the Ising model. Building on earlier work by Dyer and Greenhill (2000) and Bulatov and Grohe (2005), we completely classify the computational complexity of partition functions. Our...

### Knowledge Infusion: In Pursuit of Robustness in Artificial Intelligence

Leslie G Valiant
Endowing computers with the ability to apply commonsense knowledge with human-level performance is a primary challenge for computer science, comparable in importance to past great challenges in other fields of science such as the sequencing of the human genome. The right approach to this problem is still under debate. Here we shall discuss and attempt to justify one approach, that of {\it knowledge infusion}. This approach is based on the view that the fundamental objective...

### Algorithms for Message Ferrying on Mobile ad hoc Networks

Mostafa Ammar, Deeparnab Chakrabarty, Atish Das Sarma, Subrahmanyam Kalyanasundaram & Richard J. Lipton
Message Ferrying is a mobility assisted technique for working around the disconnectedness and sparsity of Mobile ad hoc networks. One of the importantquestions which arise in this context is to determine the routing of the ferry,so as to minimize the buffers used to store data at the nodes in thenetwork. We introduce a simple model to capture the ferry routingproblem. We characterize {\em stable} solutions of the system andprovide efficient approximation algorithms for the {\sc...

### Asymptotically Optimal Lower Bounds on the NIH-Multi-Party Information Complexity of the AND-Function and Disjointness

Andre Gronemeier
Here we prove an asymptotically optimal lower bound on the information complexity of the $k$-party disjointness function with the unique intersection promise, an important special case of the well known disjointness problem, and the AND$_k$-function in the number in the hand model. Our $\Omega(n/k)$ bound for disjointness improves on an earlier $\Omega(n/(k \log k))$ bound by Chakrabarti {\it et al.}~(2003), who obtained an asymptotically tight lower bound for one-way protocols, but failed to do so...

### The Covering and Boundedness Problems for Branching Vector Addition Systems

Stéphane Demri, Marcin Jurdzinski, Oded Lachish & Ranko Lazic
The covering and boundedness problems for branching vector addition systems are shown complete for doubly-exponential time.

### Polynomial Kernelizations for MIN F^+Pi_1 and MAX NP

Stefan Kratsch
The relation of constant-factor approximability to fixed-parameter tractability and kernelization is a long-standing open question. We prove that two large classes of constant-factor approximable problems, namely~$\textsc{MIN F}^+\Pi_1$ and~$\textsc{MAX NP}$, including the well-known subclass~$\textsc{MAX SNP}$, admit polynomial kernelizations for their natural decision versions. This extends results of Cai and Chen (JCSS 1997), stating that the standard parameterizations of problems in~$\textsc{MAX SNP}$ and~$\textsc{MIN F}^+\Pi_1$ are fixed-parameter tractable, and complements recent research on problems that do not admit...

### On the Borel Inseparability of Game Tree Languages

Szczepan Hummel, Henryk Michalewski & Damian Niwinski
The game tree languages can be viewed as an automata-theoretic counterpart of parity games on graphs. They witness the strictness of the index hierarchy of alternating tree automata, as well as the fixed-point hierarchy over binary trees. We consider a game tree language of the first non-trivial level, where Eve can force that 0 repeats from some moment on, and its dual, where Adam can force that 1 repeats from some moment on. Both these...

### Economical Caching

Matthias Englert, Heiko Röglin, Jacob Spönemann & Berthold Vöcking
We study the management of buffers and storages in environments with unpredictably varying prices in a competitive analysis. In the economical caching problem, there is a storage with a certain capacity. For each time step, an online algorithm is given a price from the interval $[1,\alpha]$, a consumption, and possibly a buying limit. The online algorithm has to decide the amount to purchase from some commodity, knowing the parameter $\alpha$ but without knowing how the...

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