569 research outputs found
Cyclic proof systems for modal fixpoint logics
This thesis is about cyclic and ill-founded proof systems for modal fixpoint logics, with and without explicit fixpoint quantifiers.Cyclic and ill-founded proof-theory allow proofs with infinite branches or paths, as long as they satisfy some correctness conditions ensuring the validity of the conclusion. In this dissertation we design a few cyclic and ill-founded systems: a cyclic one for the weak Grzegorczyk modal logic K4Grz, based on our explanation of the phenomenon of cyclic companionship; and ill-founded and cyclic ones for the full computation tree logic CTL* and the intuitionistic linear-time temporal logic iLTL. All systems are cut-free, and the cyclic ones for K4Grz and iLTL have fully finitary correctness conditions.Lastly, we use a cyclic system for the modal mu-calculus to obtain a proof of the uniform interpolation property for the logic which differs from the original, automata-based one
An Infinite Needle in a Finite Haystack: Finding Infinite Counter-Models in Deductive Verification
First-order logic, and quantifiers in particular, are widely used in
deductive verification. Quantifiers are essential for describing systems with
unbounded domains, but prove difficult for automated solvers. Significant
effort has been dedicated to finding quantifier instantiations that establish
unsatisfiability, thus ensuring validity of a system's verification conditions.
However, in many cases the formulas are satisfiable: this is often the case in
intermediate steps of the verification process. For such cases, existing tools
are limited to finding finite models as counterexamples. Yet, some quantified
formulas are satisfiable but only have infinite models. Such infinite
counter-models are especially typical when first-order logic is used to
approximate inductive definitions such as linked lists or the natural numbers.
The inability of solvers to find infinite models makes them diverge in these
cases. In this paper, we tackle the problem of finding such infinite models.
These models allow the user to identify and fix bugs in the modeling of the
system and its properties. Our approach consists of three parts. First, we
introduce symbolic structures as a way to represent certain infinite models.
Second, we describe an effective model finding procedure that symbolically
explores a given family of symbolic structures. Finally, we identify a new
decidable fragment of first-order logic that extends and subsumes the
many-sorted variant of EPR, where satisfiable formulas always have a model
representable by a symbolic structure within a known family. We evaluate our
approach on examples from the domains of distributed consensus protocols and of
heap-manipulating programs. Our implementation quickly finds infinite
counter-models that demonstrate the source of verification failures in a simple
way, while SMT solvers and theorem provers such as Z3, cvc5, and Vampire
diverge
Clones over Finite Sets and Minor Conditions
Achieving a classification of all clones of operations over a finite set is one of the goals at the heart of universal algebra. In 1921 Post provided a full description of the lattice of all clones over a two-element set. However, over the following years, it has been shown that a similar classification seems arduously reachable even if we only focus on clones over three-element sets: in 1959 Janov and Mučnik proved that there exists a continuum of clones over a k-element set for every k > 2. Subsequent research in universal algebra therefore focused on understanding particular aspects of clone lattices over finite domains. Remarkable results in this direction are the description of maximal and minimal clones. One might still hope to classify all operation clones on finite domains up to some equivalence relation so that equivalent clones share many of the properties that are of interest in universal algebra.
In a recent turn of events, a weakening of the notion of clone homomorphism was introduced: a minor-preserving map from a clone C to D is a map which preserves arities and composition with projections. The minor-equivalence relation on clones over finite sets gained importance both in universal algebra and in computer science: minor-equivalent clones satisfy the same set identities of the form f(x_1,...,x_n) = g(y_1,...,y_m), also known as minor-identities. Moreover, it was proved that the complexity of the CSP of a finite structure A only depends on the set of minor-identities satisfied by the polymorphism clone of A. Throughout this dissertation we focus on the poset that arises by considering clones over a three-element set with the following order: we write C ≤_{m} D if there exist a minor-preserving map from C to D. It has been proved that ≤_{m} is a preorder; we call the poset arising from ≤_{m} the pp-constructability poset.
We initiate a systematic study of the pp-constructability poset. To this end, we distinguish two cases that are qualitatively distinct: when considering clones over a finite set A, one can either set a boundary on the cardinality of A, or not. We denote by P_n the pp-constructability poset restricted to clones over a set A such that |A|=n and by P_{fin} we denote the whole pp-constructability poset, i.e., we only require A to be finite. First, we prove that P_{fin} is a semilattice and that it has no atoms. Moreover, we provide a complete description of P_2 and describe a significant part of P_3: we prove that P_3 has exactly three submaximal elements and present a full description of the ideal generated by one of these submaximal elements. As a byproduct, we prove that there are only countably many clones of self-dual operations over {0,1,2} up to minor-equivalence
The logic of separation logic: Models and proofs
The standard semantics of separation logic is restricted to finite heaps. This restriction already gives rise to a logic which does not satisfy compactness, hence it does not allow for an effective, sound and complete axiomatization. In this paper we therefore study both the general model theory and proof theory of the separation logic of finite and infinite heaps over arbitrary (first-order) models. We show that we can express in the resulting logic finiteness of the models and the existence of both countably infinite and uncountable models. We further show that a sound and complete sequent calculus still can be obtained by restricting the second-order quantification over heaps to first-order definable heaps
Algebraizable Weak Logics
We extend the standard framework of abstract algebraic logic to the setting
of logics which are not closed under uniform substitution. We introduce the
notion of weak logics as consequence relations closed under limited forms of
substitutions and we give a modified definition of algebraizability that
preserves the uniqueness of the equivalent algebraic semantics of algebraizable
logics. We provide several results for this novel framework, in particular a
connection between the algebraizability of a weak logic and the standard
algebraizability of its schematic fragment. We apply this framework to the
context of logics defined over team semantics and we show that the classical
version of inquisitive and dependence logic is algebraizable, while their
intuitionistic versions are not
LIPIcs, Volume 261, ICALP 2023, Complete Volume
LIPIcs, Volume 261, ICALP 2023, Complete Volum
Epistemic Logics of Structured Intensional Groups
Epistemic logics of intensional groups lift the assumption that membership in
a group of agents is common knowledge. Instead of being represented directly as
a set of agents, intensional groups are represented by a property that may
change its extension from world to world. Several authors have considered
versions of the intensional group framework where group-specifying properties
are articulated using structured terms of a language, such as the language of
Boolean algebras or of description logic. In this paper we formulate a general
semantic framework for epistemic logics of structured intensional groups,
develop the basic theory leading to completeness-via-canonicity results, and
show that several frameworks presented in the literature correspond to special
cases of the general framework.Comment: In Proceedings TARK 2023, arXiv:2307.0400
Interpolation Is (Not Always) Easy to Spoil
We study a version of the Craig interpolation theorem as formulated in the framework of the theory of institutions. This formulation proved crucial in the development of a number of key results concerning foundations of software specification and formal development. We investigate preservation of interpolation under extensions of institutions by new models and sentences. We point out that some interpolation properties remain stable under such extensions, even if quite arbitrary new models or sentences are permitted. We give complete characterisations of such situations for institution extensions by new models, by new sentences, as well as by new models and sentences, respectively
Profinite lambda-terms and parametricity
Combining ideas coming from Stone duality and Reynolds parametricity, we
formulate in a clean and principled way a notion of profinite lambda-term
which, we show, generalizes at every type the traditional notion of profinite
word coming from automata theory. We start by defining the Stone space of
profinite lambda-terms as a projective limit of finite sets of usual
lambda-terms, considered modulo a notion of equivalence based on the finite
standard model. One main contribution of the paper is to establish that,
somewhat surprisingly, the resulting notion of profinite lambda-term coming
from Stone duality lives in perfect harmony with the principles of Reynolds
parametricity. In addition, we show that the notion of profinite lambda-term is
compositional by constructing a cartesian closed category of profinite
lambda-terms, and we establish that the embedding from lambda-terms modulo
beta-eta-conversion to profinite lambda-terms is faithful using Statman's
finite completeness theorem. Finally, we prove that the traditional Church
encoding of finite words into lambda-terms can be extended to profinite words,
and leads to a homeomorphism between the space of profinite words and the space
of profinite lambda-terms of the corresponding Church type
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