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08:45-09:15 Session 37: VSL Opening
Location: EI, EI 7 + EI 9, EI 10 + FH, Hörsaal 1
Welcome Address by the Rector
Welcome Address by the Organizers
SPEAKER: Matthias Baaz
VSL Opening
SPEAKER: Dana Scott
09:15-10:15 Session 38: VSL Keynote Talk
Location: EI, EI 7 + EI 9, EI 10 + FH, Hörsaal 1
VSL Keynote Talk: Computational Ideas and the Theory of Evolution

ABSTRACT. Covertly computational insights have influenced the Theory of Evolution from the very start. I shall discuss recent work on some central problems in Evolution that was inspired and informed by computational ideas. Considerations about the performance of genetic algorithms led to a novel theory of the role of sex in Evolution based on the concept of mixability, the population genetic equations describing the evolution of a species can be reinterpreted as a repeated game between genes played through the multiplicative update algorithm, while a theorem on satisfiability can shed light on the emergence of complex adaptations.

10:15-10:45Coffee Break
10:45-13:00 Session 38D
Location: Informatikhörsaal
Implicational Relevance Logic is 2-ExpTime-Complete

ABSTRACT. We show that provability in the implicational fragment of relevance logic is complete for doubly exponential time, using reductions to and from coverability in branching vector addition systems.

An Implicit Characterization of the Polynomial-Time Decidable Sets by Cons-Free Rewriting
SPEAKER: unknown

ABSTRACT. A constructor rewriting system $R$ is said to characterize a language $D \subseteq \{ 0, 1 \}^\{< \infty}$ if there are designated function symbols $f_0$ and $\mathrm{true}$ such that for every constructor term $s$ that represents a string $\mathtt{s} \in \{0,1\}^{< \infty}$, we have $f_0(s) \rightarrow^* \mathrm{true}$ iff $\mathtt{s} \in D$. A class $\mathfrak{R}$ of rewriting systems is said to characterize a class $\mathfrak{D}$ of languages if every language in $\mathfrak{D}$ is characterized by a system in $\mathfrak{R}$ and every element of $\mathfrak{R}$ characterizes a language in $\mathfrak{D}$. We define the class of \emph{constrained cons-free} rewriting systems and show that this class characterizes $P$, the set of languages decidable in polynomial time on a deterministic Turing machine. The main novelty of the characterization is that it allows very liberal properties of term rewriting, in particular non-deterministic evaluation: no reduction strategy is enforced, and systems are allowed to be non-confluent.

Unification and Logarithmic Space
SPEAKER: Marc Bagnol

ABSTRACT. We present an algebraic characterization of the complexity classes Logspace and NLogspace, using an algebra with a composition law based on unification. This new bridge between unification and complexity classes is inspired from proof theory and more specifically linear logic and Geometry of Interaction.

We show how unification can be used to build a model of computation by means of specific subalgebras associated to finite permutations groups.

We then prove that the complexity of deciding whenever an observation (the algebraic counterpart of a program) accepts a word can be decided within logarithmic space. We also show that the construction can naturally be related to pointer machines, an intuitive way of understanding logarithmic space computing.

Automated Complexity Analysis Based on Context-Sensitive Rewriting
SPEAKER: unknown

ABSTRACT. In this paper we present a simple technique for analysing the runtime complexity of rewrite systems. In complexity analysis many techniques are based on reduction orders. We show how the monotonicity condition for orders can be weakened by using the notion of context-sensitive rewriting. The presented technique is very easy to implement and has been integrated in the Tyrolean Complexity Tool. We provide ample experimental data for assessing the viability of our method.

Automatic Evaluation of Context-Free Grammars (System Description)
SPEAKER: unknown

ABSTRACT. We implement an online judge for context-free grammars. Our system contains a list of problems describing formal languages, and asking for grammars generating them. A submitted proposal grammar receives a verdict of acceptance or rejection depending on whether the judge determines that it is equivalent to the reference solution grammar provided by the problem setter. Since equivalence of context-free grammars is an undecidable problem, we consider a maximum length L and only test equivalence of the generated languages up to words of length L. This length restriction is very often sufficient for the well-meant submissions. Since this restricted problem is still NP-complete, we design and implement methods based on hashing, SAT, and automata that perform well in practice.

13:00-14:30Lunch Break
14:30-16:00 Session 41C
Location: Informatikhörsaal
Process types as a descriptive tool for distributed protocols
Applicative May- and Should-Simulation in the Call-by-Value Lambda Calculus with AMB
SPEAKER: David Sabel

ABSTRACT. Motivated by the question whether sound and expressive applicative similarities for program calculi with should-convergence exist, this paper investigates expressive applicative similarities for the untyped call-by-value lambda-calculus extended with McCarthy's ambiguous choice operator amb. Soundness of the applicative similarities w.r.t. contextual equivalence based on may- and should-convergence is proved by adapting Howe's method to should-convergence. As usual for nondeterministic calculi, similarity is not complete w.r.t. contextual equivalence which requires a rather complex counter example as a witness.Also the call-by-value lambda-calculus with the weaker nondeterministic construct erratic choice is analyzed and sound applicative similarities are provided. This justifies the expectation that also for more expressive and call-by-need higher-order calculi there are sound and powerful similarities for should-convergence.

16:00-16:30Coffee Break
16:30-18:15 Session 42D
Location: Informatikhörsaal
A Coinductive Confluence Proof for Infinitary Lambda-Calculus.

ABSTRACT. We give a coinductive proof of confluence, up to equivalence of root-active subterms, of infinitary lambda-calculus. We also show confluence of Böhm reduction (with respect to root-active terms) in infinitary lambda-calculus. In contrast to previous proofs, our proof makes heavy use of coinduction and does not employ the notion of descendants.

Confluence by Critical Pair Analysis
SPEAKER: unknown

ABSTRACT. Knuth and Bendix showed that confluence of a terminating first-order rewrite system can be reduced to the joinability of its finitely many critical pairs. We show that this is still true of a rewrite system RT ∪ RNT such that RT is terminating and RNT is a left-linear, rank non-increasing, possibly non-terminating rewrite system. Confluence can then be reduced to the joinability of the critical pairs of RT and to the existence of decreasing diagrams for the critical pairs of RT inside RNT as well as for the rigid parallel critical pairs of RNT.

Proving Confluence of Term Rewriting Systems via Persistency and Decreasing Diagrams
SPEAKER: unknown

ABSTRACT. Decreasing diagrams technique (van Oostrom, 1994) has been successfully used to prove confluence of rewrite systems in various ways; using rule-labelling (van Oostrom, 2008), it can be also applied directly to prove confluence of some linear term rewriting systems (TRSs) automatically. Some efforts for extending the rule-labelling are known, but non-left-linear TRSs are left beyond the scope. Two methods for automatically proving confluence of non-(left-)linear TRSs with the rule-labelling are given. The key idea of our methods is to combine the decreasing diagrams technique with persistency of confluence (Aoto & Toyama, 1997).

Conditional Confluence (System Description)

ABSTRACT. This paper describes the Conditional Confluence tool, a fully automatic confluence checker for first-order conditional term rewrite systems. The tool implements various confluence criteria that have been proposed in the literature. A simple technique is presented to test conditional critical pairs for infeasibility, which makes conditional confluence criteria more useful. Detailed experimental data is presented.