### Deciding Unambiguity and Sequentiality of Polynomially Ambiguous Min-Plus Automata

Daniel Kirsten & Sylvain Lombardy
This paper solves the unambiguity and the sequentiality problem for polynomially ambiguous min-plus automata. This result is proved through a decidable algebraic characterization involving so-called metatransitions and an application of results from the structure theory of finite semigroups. It is noteworthy that the equivalence problem is known to be undecidable for polynomially ambiguous automata.

### 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...

### Mediating for Reduction (on Minimizing Alternating Büchi Automata)

Parosh A. Abdulla, Yu-Fang Chen, Lukas Holik & Tomas Vojnar
We propose a new approach for minimizing alternating B\"uchi automata (ABA). The approach is based on the so called \emph{mediated equivalence} on states of ABA, which is the maximal equivalence contained in the so called \emph{mediated preorder}. Two states $p$ and $q$ can be related by the mediated preorder if there is a~\emph{mediator} (mediating state) which forward simulates $p$ and backward simulates $q$. Under some further conditions, letting a computation on some word jump from...

### Covering of ordinals

Laurent Braud
The paper focuses on the structure of fundamental sequences of ordinals smaller than $\e$. A first result is the construction of a monadic second-order formula identifying a given structure, whereas such a formula cannot exist for ordinals themselves. The structures are precisely classified in the pushdown hierarchy. Ordinals are also located in the hierarchy, and a direct presentation is given.

### Automata and temporal logic over arbitrary linear time

Julien Cristau
Linear temporal logic was introduced in order to reason about reactive systems. It is often considered with respect to infinite words, to specify the behaviour of long-running systems. One can consider more general models for linear time, using words indexed by arbitrary linear orderings. We investigate the connections between temporal logic and automata on linear orderings, as introduced by Bruyere and Carton. We provide a doubly exponential procedure to compute from any LTL formula with...

### 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.

### Subexponential Algorithms for Partial Cover Problems

Fedor V. Fomin, Daniel Lokshtanov, Venkatesh Raman & Saket Saurabh
Partial Cover problems are optimization versions of fundamental and well studied problems like {\sc Vertex Cover} and {\sc Dominating Set}. Here one is interested in covering (or dominating) the maximum number of edges (or vertices) using a given number ($k$) of vertices, rather than covering all edges (or vertices). In general graphs, these problems are hard for parameterized complexity classes when parameterized by $k$. It was recently shown by Amini et. al. [{\em FSTTCS 08}\,]...

### Implicit Branching and Parameterized Partial Cover Problems (Extended Abstract)

Omid Amini, Fedor Fomin & Saket Saurabh
Covering problems are fundamental classical problems in optimization, computer science and complexity theory. Typically an input to these problems is a family of sets over a finite universe and the goal is to cover the elements of the universe with as few sets of the family as possible. The variations of covering problems include well known problems like Set Cover, Vertex Cover, Dominating Set and Facility Location to name a few. Recently there has been...

### Solvency Games

Noam Berger, Nevin Kapur, Leonard Schulman & Vijay Vazirani
We study the decision theory of a maximally risk-averse investor --- one whose objective, in the face of stochastic uncertainties, is to minimize the probability of ever going broke. With a view to developing the mathematical basics of such a theory, we start with a very simple model and obtain the following results: a characterization of best play by investors; an explanation of why poor and rich players may have different best strategies; an explanation...

### Graph Games on Ordinals

Julien Cristau & Florian Horn
We consider an extension of Church\'s synthesis problem to ordinals by adding limit transitions to graph games. We consider game arenas where these limit transitions are defined using the sets of cofinal states. In a previous paper, we have shown that such games of ordinal length are determined and that the winner problem is \pspace-complete, for a subclass of arenas where the length of plays is always smaller than $\omega^\omega$. However, the proof uses a...

### A new approach to the planted clique problem

Alan Frieze & Ravi Kannan
We study the problem of finding a large planted clique in the random graph $G_{n,1/2}$. We reduce the problem to that of maximising a three dimensional tensor over the unit ball in $n$ dimensions. This latter problem has not been well studied and so we hope that this reduction will eventually lead to an improved solution to the planted clique problem.

### The unfolding of general Petri nets

Jonathan Hayman & Glynn Winskel
The unfolding of (1-)safe Petri nets to occurrence nets is well understood. There is a universal characterization of the unfolding of a safe net which is part and parcel of a coreflection from the category of occurrence nets to the category of safe nets. The unfolding of general Petri nets, nets with multiplicities on arcs whose markings are multisets of places, does not possess a directly analogous universal characterization, essentially because there is an implicit...

### The Complexity of Tree Transducer Output Languages

Kazuhiro Inaba & Sebastian Maneth
Two complexity results are shown for the output languages generated by compositions of macro tree transducers. They are in $\NSPACE(n)$ and hence are context-sensitive, and the class is NP-complete.

### STCON in Directed Unique-Path Graphs

Sampath Kannan, Sanjeev Khanna & Sudeepa Roy
We study the problem of space-efficient polynomial-time algorithms for {\em directed st-connectivity} (STCON). Given a directed graph $G$, and a pair of vertices $s, t$, the STCON problem is to decide if there exists a path from $s$ to $t$ in $G$. For general graphs, the best polynomial-time algorithm for STCON uses space that is only slightly sublinear. However, for special classes of directed graphs, polynomial-time poly-logarithmic-space algorithms are known for STCON. In this paper,...

### Dynamic matrix rank with partial lookahead

Telikepalli Kavitha
We consider the problem of maintaining information about the rank of a matrix $M$ under changes to its entries. For an $n \times n$ matrix $M$, we show an amortized upper bound of $O(n^{\omega-1})$ arithmetic operations per change for this problem, where $\omega < 2.376$ is the exponent for matrix multiplication, under the assumption that there is a {\em lookahead} of up to $\Theta(n)$ locations. That is, we know up to the next $\Theta(n)$ locations...

### 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...

### 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...

### Approximating Acyclicity Parameters of Sparse Hypergraphs

Fedor V. Fomin, Petr A. Golovach & Dimitrios M. Thilikos
The notions of hypertree width and generalized hypertree width were introduced by Gottlob, Leone, and Scarcello (PODS'99, PODS'01) in order to extend the concept of hypergraph acyclicity. These notions were further generalized by Grohe and Marx in SODA'06, who introduced the fractional hypertree width of a hypergraph. All these width parameters on hypergraphs are useful for extending tractability of many problems in database theory and artificial intelligence. Computing each of these width parameters is known...

### Equations over Sets of Natural Numbers with Addition Only

Artur Jez & Alexander Okhotin
Systems of equations of the form $X=YZ$ and $X=C$ are considered, in which the unknowns are sets of natural numbers, $+$'' denotes pairwise sum of sets $S+T=\ensuremath{ \{ m+n \: | \: m \in S, \; n \in T \} }$, and $C$ is an ultimately periodic constant. It is shown that such systems are computationally universal, in the sense that for every recursive (r.e., co-r.e.) set $S \subseteq \mathbb{N}$ there exists a system with...

### 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...

### 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 Generalization of Nemhauser and Trotter's Local Optimization Theorem

Michael R. Fellows, Jiong Guo, Hannes Moser & Rolf Niedermeier
The Nemhauser-Trotter local optimization theorem applies to the NP-hard \textsc{Vertex Cover} problem and has applications in approximation as well as parameterized algorithmics. We present a framework that generalizes Nemhauser and Trotter's result to vertex deletion and graph packing problems, introducing novel algorithmic strategies based on purely combinatorial arguments (not referring to linear programming as the Nemhauser-Trotter result originally did). We exhibit our framework using a generalization of \textsc{Vertex Cover}, called \textrm{\sc Bounded-Degree Deletion}, that has...

### 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...

### Randomness on Computable Probability Spaces - A Dynamical Point of View

Peter Gacs, Mathieu Hoyrup & Cristobal Rojas
We extend the notion of randomness (in the version introduced by Schnorr) to computable Probability Spaces and compare it to a \emph{dynamical} notion of randomness: typicality. Roughly, a point is \emph{typical} for some dynamic, if it follows the statistical behavior of the system (Birkhoff's pointwise ergodic theorem). We prove that a point is Schnorr random if and only if it is typical for every \emph{mixing} computable dynamics. To prove the result we develop some tools...

### Polynomial-Time Approximation Schemes for Subset-Connectivity Problems in Bounded-Genus Graphs

Glencora Borradaile, Erik D. Demaine & Siamak Tazari
We present the first polynomial-time approximation schemes (PTASes) for the following subset-connectivity problems in edge-weighted graphs of bounded genus: Steiner tree, low-connectivity survivable-network design, and subset TSP. The schemes run in $O(n \log n)$ time for graphs embedded on both orientable and non-orientable surfaces. This work generalizes the PTAS frameworks of Borradaile, Klein, and Mathieu (2007 and 2006) from planar graphs to bounded-genus graphs: any future problems shown to admit the required structure theorem for...

• 2009
134

• Text
134