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The algebra of binary relations provides union and composition as basic operators, with the empty set as neutral element for union and the identity relation as neutral element for composition. The basic algebra can be enriched with additional features. We consider the diversity relation, the full relation, intersection, set difference, projection, coprojection, converse, and transitive closure. It...
We give a model of dependent type theory with one univalent universe and propositional truncation interpreting a type as a stack, generalizing the groupoid model of type theory. As an application, we show that countable choice cannot be proved in dependent type theory with one univalent universe and propositional truncation.
A successor-invariant first-order formula is a formula that has access to an auxiliary successor relation on a structure's universe, but the model relation is independent of the particular interpretation of this relation. It is well known that successor-invariant formulas are more expressive on finite structures than plain first-order formulas without a successor relation. This naturally raises the...
In this paper I distinguish two (pre)congruence requirements for semantic equivalences and preorders on processes given as closed terms in a system description language with a recursion construct. A lean congruence preserves equivalence when replacing closed subexpressions of a process by equivalent alternatives. A full congruence moreover allows replacement within a recursive specification of subexpressions...
Stable event structures, and their duality with prime algebraic domains (arising as partial orders of configurations), are a landmark of concurrency theory, providing a clear characterisation of causality in computations. They have been used for defining a concurrent semantics of several formalisms, from Petri nets to linear graph rewriting systems, which in turn lay at the basis of many visual frameworks...
This paper studies sets of rational numbers definable by continuous Petri nets and extensions thereof. First, we identify a polynomial-time decidable fragment of existential FO(ℚ,+,<) and show that the sets of rationals definable in this fragment coincide with reachability sets of continuous Petri nets. Next, we introduce and study continuous vector addition systems with states (CVASS), which are...
We show how to derive natural deduction systems for the necessity fragment of various constructive modal logics by exploiting a pattern found in sequent calculi. The resulting systems are dual-context systems, in the style pioneered by Girard, Barber, Plotkin, Pfenning, Davies, and others. This amounts to a full extension of the Curry-Howard-Lambek correspondence to the necessity fragments of a constructive...
We present two finitary cut-free sequent calculi for the modal μ-calculus. One is a variant of Kozen's axiomatisation in which cut is replaced by a strengthening of the induction rule for greatest fixed point. The second calculus derives annotated sequents in the style of Stirling's ‘tableau proof system with names’ (2014) and features a generalisation of the ν-regeneration rule that allows discharging...
We introduce fibred type-theoretic fibration categories which are fibred categories between categorical models of Martin-Löf type theory. Fibred type-theoretic fibration categories give a categorical description of logical predicates for identity types. As an application, we show a relational parametricity result for homotopy type theory. As a corollary, it follows that every closed term of type of...
We present a weakest-precondition-style calculus for reasoning about the expected values (pre-expectations) of mixed-sign unbounded random variables after execution of a probabilistic program. The semantics of a while-loop is defined as the limit of iteratively applying a functional to a zero-element just as in the traditional weakest pre-expectation calculus, even though a standard least fixed point...
We introduce a geometry of interaction model for Mazza's multiport interaction combinators, a graph-theoretic formalism which is able to faithfully capture concurrent computation as embodied by process algebras like the π-calculus. The introduced model is based on token machines in which not one but multiple tokens are allowed to traverse the underlying net at the same time. We prove soundness and...
We study Abramsky's applicative bisimilarity abstractly, in the context of call-by-value λ-calculi with algebraic effects. We first of all endow a computational λ-calculus with a monadic operational semantics. We then show how the theory of relators provides precisely what is needed to generalise applicative bisimilarity to such a calculus, and to single out those monads and relators for which applicative...
We present fully abstract encodings of the call-byname λ-calculus into HOcore, a minimal higher-order process calculus with no name restriction. We consider several equivalences on the λ-calculus side—normal-form bisimilarity, applicative bisimilarity, and contextual equivalence—that we internalize into abstract machines in order to prove full abstraction.
We present an overview of some recent work on the quantitative semantics of the λ-calculus. Our starting point is the fundamental degenerate model of linear logic, the relational model MRel. We show that three quantitative semantics of the simply-typed λ-calculus are equivalent: the relational semantics, HO/N game semantics, and the Taylor expansion semantics. We then consider two recent generalisations...
During the past several decades, the database theory community has successfully investigated several different facets of the principles of database systems, including the development of various data models, the systematic exploration of the expressive power of database query languages, and, more recently, the study of the foundations of information integration via schema mappings. For the most part,...
Quantitative algebras (QAs) are algebras over metric spaces defined by quantitative equational theories as introduced by us in 2016. They provide the mathematical foundation for metric semantics of probabilistic, stochastic and other quantitative systems. This paper considers the issue of axiomatizability of QAs. We investigate the entire spectrum of types of quantitative equations that can be used...
Pebble games are a powerful tool in the study of finite model theory, constraint satisfaction and database theory. Monads and comonads are basic notions of category theory which are widely used in semantics of computation and in modern functional programming. We show that existential k-pebble games have a natural comonadic formulation. Winning strategies for Duplicator in the k-pebble game for structures...
A structure enjoys the Herbrand property if, whenever it satisfies an equality between some terms, these terms are unifiable. On such structures the expressive power of equalities becomes trivial, as their semantic satisfiability is reduced to a purely syntactic check.
We present an extension of the computation system and logic of the Nuprl proof assistant with intuitionistic principles, namely versions of Brouwer's bar induction principle, which is equivalent to transfinite induction. We have substantially extended the formalization of Nuprl's type theory within the Coq proof assistant to show that two such bar induction principles are valid w.r.t. Nuprl's semantics...
We introduce the logic FOCN(ℙ) which extends first-order logic by counting and by numerical predicates from a set ℙ, and which can be viewed as a natural generalisation of various counting logics that have been studied in the literature.
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