Showing papers presented at "Computer Science Logic in 2006"

Automata and logics for words and trees over an infinite alphabet

Abstract: In a data word or a data tree each position carries a label from a finite alphabet and a data value from some infinite domain. These models have been considered in the realm of semistructured data, timed automata and extended temporal logics. This paper survey several know results on automata and logics manipulating data words and data trees, the focus being on their relative expressive power and decidability.

Differential interaction nets

Abstract: We introduce interaction nets for a fragment of the differential lambda-calculus and exhibit in this framework a new symmetry between the of course and the why not modalities of linear logic, which is completely similar to the symmetry between the tensor and par connectives of linear logic. We use algebraic intuitions for introducing these nets and their reduction rules, and then we develop two correctness criteria (weak typability and acyclicity) and show that they guarantee strong normalization. Finally, we outline the correspondence between this interaction nets formalism and the resource lambda-calculus.

Solving games without determinization

Abstract: The synthesis of reactive systems requires the solution of two-player games on graphs with ω-regular objectives. When the objective is specified by a linear temporal logic formula or nondeterministic Buchi automaton, then previous algorithms for solving the game require the construction of an equivalent deterministic automaton. However, determinization for automata on infinite words is extremely complicated, and current implementations fail to produce deterministic automata even for relatively small inputs. We show how to construct, from a given nondeterministic Buchi automaton, an equivalent nondeterministic parity automaton $\ensuremath {\cal P}$ that is good for solving games with objective $\ensuremath {\cal P}$. The main insight is that a nondeterministic automaton is good for solving games if it fairly simulates the equivalent deterministic automaton. In this way, we omit the determinization step in game solving and reactive synthesis. The fact that our automata are nondeterministic makes them surprisingly simple, amenable to symbolic implementation, and allows an incremental search for winning strategies.

Algorithms for omega-regular games with imperfect information

Abstract: We study observation-based strategies for two-player turn-based games on graphs with omega-regular objectives. An observation-based strategy relies on imperfect information about the history of a play, namely, on the past sequence of observations. Such games occur in the synthesis of a controller that does not see the private state of the plant. Our main results are twofold. First, we give a fixed-point algorithm for computing the set of states from which a player can win with a deterministic observation-based strategy for any omega-regular objective. The fixed point is computed in the lattice of antichains of state sets. This algorithm has the advantages of being directed by the objective and of avoiding an explicit subset construction on the game graph. Second, we give an algorithm for computing the set of states from which a player can win with probability 1 with a randomized observation-based strategy for a Buchi objective. This set is of interest because in the absence of perfect information, randomized strategies are more powerful than deterministic ones. We show that our algorithms are optimal by proving matching lower bounds.

MSO queries on tree decomposable structures are computable with linear delay

Abstract: Linear-Delaylin is the class of enumeration problems computable in two steps: the first step is a precomputation in linear time in the size of the input and the second step computes successively all the solutions with a delay between two consecutive solutions y1 and y2 that is linear in |y2| We prove that evaluating a fixed monadic second order (MSO) query $\varphi(\bar{X})$ (ie computing all the tuples that satisfy the MSO formula) in a binary tree is a Linear-Delaylin problem More precisely, we show that given a binary tree T and a tree automaton Γ representing an MSO query $\varphi(\bar{X})$, we can evaluate Γ on T with a preprocessing in time and space complexity O(|Γ|3|T|) and an enumeration phase with a delay O(|S|) and space O(max|S|) where |S| is the size of the next solution and max|S| is the size of the largest solution We introduce a new kind of algorithm with nice complexity properties for some algebraic operations on enumeration problems In addition, we extend the precomputation (with the same complexity) such that the ith (with respect to a certain order) solution S is produced directly in time O(|S|log(|T|)) Finally, we generalize these results to bounded treewidth structures

Functorial boxes in string diagrams

Abstract: String diagrams were introduced by Roger Penrose as a handy notation to manipulate morphisms in a monoidal category. In principle, this graphical notation should encompass the various pictorial systems introduced in proof-theory (like Jean-Yves Girard’s proof-nets) and in concurrency theory (like Robin Milner’s bigraphs). This is not the case however, at least because string diagrams do not accomodate boxes — a key ingredient in these pictorial systems. In this short tutorial, based on our accidental rediscovery of an idea by Robin Cockett and Robert Seely, we explain how string diagrams may be extended with a notion of functorial box depicting a functor transporting an inside world (its source category) to an outside world (its target category). We expose two elementary applications of the notation: first, we characterize graphically when a faithful balanced monoidal functor F:ℂ \(\longrightarrow\) \(\mathbb{D}\) transports a trace operator from the category \(\mathbb{D}\) to the category ℂ, and exploit this to construct well-behaved fixpoint operators in cartesian closed categories generated by models of linear logic; second, we explain how the categorical semantics of linear logic induces that the exponential box of proof-nets decomposes as two enshrined boxes.

Satisfiability and finite model property for the alternating-time µ-calculus

Abstract: This paper presents a decision procedure for the alternating-time μ-calculus. The algorithm is based on a representation of alternating-time formulas as automata over concurrent game structures. We show that language emptiness of these automata can be checked in exponential time. The complexity of our construction meets the known lower bounds for deciding the satisfiability of the classic μ-calculus. It follows that the satisfiability problem is EXPTIME-complete for the alternating-time μ-calculus.

Axiomatisations of functional dependencies in the presence of records, lists, sets and multisets

Abstract: We investigate functional dependencies in databases that support complex values such as records, lists, sets anu multisets. Therefore, an abstract algebraic framework is proposed that classifies data models according to the underlying types they support. This allows to emphasise the impact of the data types rather than the specifics of a particular data model.The main results are finite, minimal, sound and complete sets of inference rules for the implication of functional dependencies in the presence of records and all combinations of lists, sets and multisets. The inference rules are similar to Armstrong's original axioms for the relational data model, thanks to the algebraic framework. The completeness result, however, requires a deep analysis in the case of sets and, in particular, multisets.

Logical omniscience via proof complexity

Abstract: The Hintikka-style modal logic approach to knowledge contains a well-known defect of logical omniscience, i.e., the unrealistic feature that an agent knows all logical consequences of her assumptions. In this paper, we suggest the following Logical Omniscience Test (LOT): an epistemic system E is not logically omniscient if for any valid in E knowledge assertion $\mathcal{A}$ of type ‘Fis known,’ there is a proof of F in E, the complexity of which is bounded by some polynomial in the length of $\mathcal{A}$. We show that the usual epistemic modal logics are logically omniscient (modulo some common complexity assumptions). We also apply LOT to evidence-based knowledge systems, which, along with the usual knowledge operator Ki(F) (‘agent iknows F’), contain evidence assertions t:F (‘t is a justification for F’). In evidence-based systems, the evidence part is an appropriate extension of the Logic of Proofs LP, which guarantees that the collection of evidence terms t is rich enough to match modal logic. We show that evidence-based knowledge systems are logically omniscient w.r.t. the usual knowledge and are not logically omniscient w.r.t. evidence-based knowledge.

Decidable theories of the ordering of natural numbers with unary predicates

Abstract: Expansions of the natural number ordering by unary predicates are studied, using logics which in expressive power are located between first-order and monadic second-order logic. Building on the model-theoretic composition method of Shelah, we give two characterizations of the decidable theories of this form, in terms of effectiveness conditions on two types of “homogeneous sets”. We discuss the significance of these characterizations, show that the first-order theory of successor with extra predicates is not covered by this approach, and indicate how analogous results are obtained in the semigroup theoretic and the automata theoretic framework.

Categorical proof theory of classical propositional calculus

Abstract: We investigate semantics for classical proof based on the sequent calculus. We show that the propositional connectives are not quite well-behaved from a traditional categorical perspective, and give a more refined, but necessarily complex, analysis of how connectives may be characterised abstractly. Finally we explain the consequences of insisting on more familiar categorical behaviour.

Concurrent games with tail objectives

Abstract: We study infinite stochastic games played by two-players over a finite state space, with objectives specified by sets of infinite traces. The games are concurrent (players make moves simultaneously and independently), stochastic (the next state is determined by a probability distribution that depends on the current state and chosen moves of the players) and infinite (proceeds for infinite number of rounds). The analysis of concurrent stochastic games can be classified into: quantitative analysis, analyzing the optimum value of the game; and qualitative analysis, analyzing the set of states with optimum value 1. We consider concurrent games with tail objectives, i.e., objectives that are independent of the finite-prefix of traces, and show that the class of tail objectives are strictly richer than the ω-regular objectives. We develop new proof techniques to extend several properties of concurrent games with ω-regular objectives to concurrent games with tail objectives. We prove the positive limit-one property for tail objectives, that states for all concurrent games if the optimum value for a player is positive for a tail objective Φ at some state, then there is a state where the optimum value is 1 for Φ, for the player. We also show that the optimum values of zero-sum (strictly conflicting objectives) games with tail objectives can be related to equilibrium values of nonzero-sum (not strictly conflicting objectives) games with simpler reachability objectives. A consequence of our analysis presents a polynomial time reduction of the quantitative analysis of tail objectives to the qualitative analysis for the sub-class of one-player stochastic games (Markov decision processes).

Separation logic for higher-order store

Abstract: Separation Logic is a sub-structural logic that supports local reasoning for imperative programs. It is designed to elegantly describe sharing and aliasing properties of heap structures, thus facilitating the verification of programs with pointers. In past work, separation logic has been developed for heaps containing records of basic data types. Languages like C or ML, however, also permit the use of code pointers. The corresponding heap model is commonly referred to as “higher-order store” since heaps may contain commands which in turn are interpreted as partial functions between heaps. In this paper we make Separation Logic and the benefits of local reasoning available to languages with higher-order store. In particular, we introduce an extension of the logic and prove it sound, including the Frame Rule that enables specifications of code to be extended by invariants on parts of the heap that are not accessed.

Intersection types and lambda models

Abstract: Invariance of interpretation by β-conversion is one of the minimal requirements for any standard model for the λ-calculus. With the intersection-type systems being a general framework for the study of semantic domains for the λ-calculus, the present paper provides a (syntactic) characterisation of the above mentioned requirement in terms of characterisation results Ibr intersection-type assignment systems.Instead of considering conversion as a whole, reduction and expansion will be considered separately. Not only for usual computational rules like β η, but also for a number of relevant restrictions of those. Characterisations will be also provided for (intersection) filter structures that are indeed λ-models.

Abstracting allocation

Abstract: We introduce a Floyd-Hoare-style framework for specification and verification of machine code programs, based on relational parametricity (rather than unary predicates) and using both step-indexing and a novel form of separation structure. This yields compositional, descriptive and extensional reasoning principles for many features of low-level sequential computation: independence, ownership transfer, unstructured control flow, first-class code pointers and address arithmetic. We demonstrate how to specify and verify the implementation of a simple memory manager and, independently, its clients in this style. The work has been fully machine-checked within the Coq proof assistant.

Visibly pushdown automata: from language equivalence to simulation and bisimulation

Abstract: We investigate the possibility of (bi)simulation-like preorder/equivalence checking on the class of visibly pushdown automata and its natural subclasses visibly BPA (Basic Process Algebra) and visibly one-counter automata. We describe generic methods for proving complexity upper and lower bounds for a number of studied preorders and equivalences like simulation, completed simulation, ready simulation, 2-nested simulation preorders/equivalences and bisimulation equivalence. Our main results are that all the mentioned equivalences and preorders are EXPTIME-complete on visibly pushdown automata, PSPACE-complete on visibly one-counter automata and P-complete on visibly BPA. Our PSPACE lower bound for visibly one-counter automata improves also the previously known DP-hardness results for ordinary one-counter automata and one-counter nets. Finally, we study regularity checking problems for visibly pushdown automata and show that they can be decided in polynomial time.

Abstracting allocation : The new new thing

Abstract: We introduce a Floyd-Hoare-style framework for specification and verification of machine code programs, based on relational parametricity (rather than unary predicates) and using both step-indexing and a novel form of separation structure. This yields compositional, descriptive and extensional reasoning principles for many features of low-level sequential computation: independence, ownership transfer, unstructured control flow, first-class code pointers and address arithmetic. We demonstrate how to specify a.nd verify the implementation of a simple memory manager and, independently, its clients in this style. The work has been fully machine-checked within the Coq proof assistant.

Semi-continuous sized types and termination

Abstract: A type-based approach to termination uses sized types: an ordinal bound for the size of a data structure is stored in its type. A recursive function over a sized type is accepted if it is visible in the type system that recursive calls occur just at a smaller size. This approach is only sound if the type of the recursive function is admissible, i.e., depends on the size index in a certain way. To explore the space of admissible functions in the presence of higher-kinded data types and impredicative polymorphism, a semantics is developed where sized types are interpreted as functions from ordinals into sets of strongly normalizing terms. It is shown that upper semi-continuity of such functions is a sufficient semantical criterion for admissibility. To provide a syntactical criterion, a calculus for semi-continuous function is developed.

The power of linear functions

Abstract: The linear lambda calculus is very weak in terms of expressive power: in particular, all functions terminate in linear time. In this paper we consider a simple extension with Booleans, natural numbers and a linear iterator. We show properties of this linear version of Godel’s System $\mathcal{T}$ and study the class of functions that can be represented. Surprisingly, this linear calculus is extremely expressive: it is as powerful as System $\mathcal{T}$

Generic models for computational effects

Abstract: A Freyd-category is a subtle generalisation of the notion of a category with finite products. It is suitable for modelling environments in call-by-value programming languages, such as the computational λ-calculus, with computational effects. We develop the theory of Freyd-categories with that in mind. We first show that any countable Lawvere theory, hence any signature of operations with countable arity subject to equations, directly generates a Freyd-category. We then give canonical, universal embeddings of Freyd-categories into closed Freyd-categories, characterised by being free cocompletions. The combination of the two constructions sends a signature of operations and equations to the Kleisli category for the monad on the category Set generated by it, thus refining the analysis of computational effects given by monads. That in turn allows a more structural analysis of the λc-calculus. Our leading examples of signatures arise from side-effects, interactive input/output and exceptions. We extend our analysis to an enriched setting in order to account for recursion and for computational effects and signatures that inherently involve it, such as partiality, nondeterminism and probabilistic nondeterminism.

Nash equilibrium for upward-closed objectives

Abstract: We study infinite stochastic games played by n-players on a finite graph with goals specified by sets of infinite traces. The games are concurrent (each player simultaneously and independently chooses an action at each round), stochastic (the next state is determined by a probability distribution depending on the current state and the chosen actions), infinite (the game continues for an infinite number of rounds), nonzero-sum (the players’ goals are not necessarily conflicting), and undiscounted. We show that if each player has an upward-closed objective, then there exists an e-Nash equilibrium in memoryless strategies, for every e>0; and exact Nash equilibria need not exist. Upward-closure of an objective means that if a set Z of infinitely repeating states is winning, then all supersets of Z of infinitely repeating states are also winning. Memoryless strategies are strategies that are independent of history of plays and depend only on the current state. We also study the complexity of finding values (payoff profile) of an e-Nash equilibrium. We show that the values of an e-Nash equilibrium in nonzero-sum concurrent games with upward-closed objectives for all players can be computed by computing e-Nash equilibrium values of nonzero-sum concurrent games with reachability objectives for all players and a polynomial procedure. As a consequence we establish that values of an e-Nash equilibrium can be computed in TFNP (total functional NP), and hence in EXPTIME.

Probability distribution for simple tautologies

Abstract: In this paper we investigate the size of the fraction of tautologies of the given length n against the number of all formulas of length n for implicational logic. We are specially interested in asymptotic behavior of this fraction. We demonstrate the relation between a number of premises of implicational formula and asymptotic probability of finding formula with this number of premises. Furthermore, we investigate the distribution of this asymptotic probabilities. Distribution for all formulas is contrasted with the same distribution for tautologies only. We prove those distributions to be so different that enable us to estimate likelihood of truth for a given long formula. Despite the fact that all discussed problems and methods in this paper are solved by mathematical means, the paper may have some philosophical impact on the understanding how much the phenomenon of truth is sporadic or frequent in random logical sentences.

Computing queries with higher-order logics

Abstract: In the present article, we study the expressive power of higher-order logics on finite relational structures or databases. First, we give a characterization of the expressive power of the fragments Σji and Πji, for each i ≥ 1 and each number of alternations of quantifier blocks j. Then, we get as a corollary the expressive power of HOi for each order i ≥ 2. From our results, as well as from the results of R. Hull and J. Su, it turns out that no higher-order logic can be complete. Even if we consider the union of higher-order logics of all natural orders, i.e., ∪i ≥ 2 HOi, we still do not get a complete logic. So, we define a logic which we call variable order logic (VO) which permits the use of untyped relation variables, i.e., variables of variable order, by allowing quantification over orders. We show that this logic is complete, though even non-recursive queries can be expressed in VO. Then we define a fragment of VO and we prove that it expresses exactly the class of r.e. queries. We finally give a characterization of the class of computable queries through a fragment of VO, which is undecidable.

Infinite state model-checking of propositional dynamic logics

Abstract: Model-checking problems for PDL (propositional dynamic logic) and its extension PDL∩ (which includes the intersection operator on programs) over various classes of infinite state systems (BPP, BPA, pushdown systems, prefix-recognizable systems) are studied. Precise upper and lower bounds are shown for the data/expression/combined complexity of these model-checking problems.

An algebraic point of view on the crane beach property

Abstract: A letter e ∈Σ is said to be neutral for a language L if it can be inserted and deleted at will in a word without affecting membership in L. The Crane Beach Conjecture, which was recently disproved, stated that any language containing a neutral letter and definable in first-order with arbitrary numerical predicates (${\bf FO}[\mathit{Arb}]$) is in fact FO [<] definable and is thus a regular, star-free language. More generally, we say that a logic or a computational model has the Crane Beach property if the only languages with neutral letter that it can define/compute are regular. We develop an algebraic point of view on the Crane Beach properties using the program over monoid formalism which has proved of importance in circuit complexity. Using recent communication complexity results we establish a number of Crane Beach results for programs over specific classes of monoids. These can be viewed as Crane Beach theorems for classes of bounded-width branching programs. We also apply this to a standard extension of FO using modular-counting quantifiers and show that the boolean closure of this logic’s Σ1 fragment has the CBP.

Acyclicity and coherence in multiplicative exponential linear logic

Abstract: We give a geometric condition that characterizes MELL proof structures whose interpretation is a clique in non-uniform coherent spaces: visible acyclicity We define the visible paths and we prove that the proof structures which have no visible cycles are exactly those whose interpretation is a clique It turns out that visible acyclicity has also nice computational properties, especially it is stable under cut reduction

On rational trees

Abstract: Rational graphs are a family of graphs defined using labelled rational transducers. Unlike automatic graphs (defined using synchronized transducers) the first order theory of these graphs is undecidable, there is even a rational graph with an undecidable first order theory. In this paper we consider the family of rational trees, that is rational graphs which are trees. We prove that first order theory is decidable for this family. We also present counter examples showing that this result cannot be significantly extended both in terms of logic and of structure.

Weak bisimulation approximants

Abstract: Bisimilarity and weak bisimilarity ≈ are canonical notions of equivalence between processes, which are defined co-inductively, but may be approached – and even reached – by their (transfinite) inductively-defined approximants ~α and ≈α. For arbitrary processes this approximation may need to climb arbitrarily high through the infinite ordinals before stabilising. In this paper we consider a simple yet well-studied process algebra, the Basic Parallel Processes (BPP), and investigate for this class of processes the minimal ordinal α such that ≈ = ≈α. The main tool in our investigation is a novel proof of Dickson’s Lemma. Unlike classical proofs, the proof we provide gives rise to a tight ordinal bound, of ωn, on the order type of non-increasing sequences of n-tuples of natural numbers. With this we are able to reduce a long-standing bound on the approximation hierarchy for weak bisimilarity ≈ over BPP, and show that ${\approx} = {\approx_{\omega^\omega}}$.

Jump from parallel to sequential proofs: multiplicatives

Abstract: We introduce a new class of multiplicative proof nets, J-proof nets, which are a typed version of Faggian and Maurel’s multiplicative L-nets. In J-proof nets, we can characterize nets with different degrees of sequentiality, by gradual insertion of sequentiality constraints. As a byproduct, we obtain a simple proof of the sequentialisation theorem.

NL-printable sets and nondeterministic Kolmogorov complexity

Abstract: P-printable sets were defined by Hartmanis and Yesha and have been investigated by several researchers. The analogous notion of L-printable sets was defined by Fortnow et al.; both P-printability and L-printability were shown to be related to notions of resource-bounded Kolmogorov complexity. Nondeterministic logspace (NL)-printability was defined by Jenner and Kirsig, but some basic questions regarding this notion were left open. In this paper we answer a question of Jenner and Kirsig by providing a machine-based characterization of the NL-printable sets.In order to relate NL-printability to resource-bounded Kolmogorov complexity, the paper introduces nondeterministic space-bounded Kolmogorov complexity. We present some of the basic properties of this notion of Kolmogorov complexity.Using similar techniques, we investigate relationships among classes between NL and UL.