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International audience; We present a formalization of proof-nets (and more generally, proofstructures) for the multiplicative-exponential fragment of linear logic, with a novel treatment of boxes: instead of integrating boxes into to the graphical structure (e.g., by adding explicit auxiliary doors, plus a mapping from boxed nodes to the main door, and ad hoc conditions on the nesting of boxes), we fix a graph morphism from the underlying graph of the proof-structure to the tree of boxes given by the nesting order. This approach allows to apply tools and notions from the theory of algebraic graph transformations, and obtain very synthetic presentations of sophisticated operations: for instance, each element of the Taylor expansion of a proof-structure is obtained by a pullback along a morphism representing a thick subtree of the tree of boxes. A treatment of cut elimination in this framework currently under development.

This special issue is devoted to some aspects of the new ideas that recently arose from the work of Thomas Ehrhard on the models of linear logic (LL) and of the λ-calculus. In some sense, the very origin of these ideas dates back to the introduction of LL in the 80s by Jean-Yves Girard. An obvious remark is that LL yielded a first logical quantitative account of the use of resources: the logical distinction between linear and non-linear formulas through the introduction of the exponential connectives. As explicitly mentioned by Girard in his first paper on the subject, the quantitative approach, to which he refers as ‘quantitative semantics,’ had a crucial influence on the birth of LL. And even though, at that time, it was given up for lack of ‘any logical justification’ (quoting the author), it contained rough versions of many concepts that were better understood, precisely introduced and developed much later, like differentiation and Taylor expansion for proofs. Around 2003, and thanks to the developments of LL and of the whole research area between logic and theoretical computer science, Ehrhard could come back to these fundamental intuitions and introduce the structure of finiteness space, allowing to reformulate this quantitative approach in a standard algebraic setting. The interpretation of LL in the category Fin of finiteness spaces and finitary relations suggested to Ehrhard and Regnier the differential extensions of LL and of the simply typed λ-calculus: Differential Linear Logic (DiLL) and the differential λ-calculus. The theory of LL proof-nets could be straightforwardly extended to DiLL, and a very natural notion of Taylor expansion of a proof-net (and of a λ-term) was introduced: an element of the Taylor expansion of the proof-net/term α is itself a (differential) proof-net/term and an approximation of α.

International audience; A Multiplicative-Exponential Linear Logic (MELL) proof-structure can be expanded into a (infinite) set of resource proof-structures: its Taylor expansion. We introduce a new criterion characterizing those sets of resource proof-structures that are part of the Taylor expansion of some MELL proof-structure, through a rewriting system acting both on resource and MELL proof-structures.

Many applications of denotational semantics, such as higher-order model checking or the complexity of normaliza-tion, rely on finite semantics for monomorphic type systems. We present two constructions of finite semantics for second-order Multiplicative-Additive Linear Logic (MALL2) and study their properties. We apply this to understand the gap in expressive power between MALL2 and its extension with type fixpoints, and to obtain an implicit characterization of regular languages in Elementary Linear Logic. Furthermore, some semantic results established here lay the groundwork for a sequel paper proposing a new approach to sub-polynomial implicit complexity.

International audience; We show that every connected Multiplicative Exponential Linear Logic (MELL) proof-structure (with or without cuts) is uniquely determined by a well-chosen element of its Taylor expansion: the one obtained by taking two copies of the content of each box. As a consequence, the relational model is injective with respect to connected MELL proof-structures.

We prove that given two cut free nets of linear logic, by means of their relational interpretations one can: 1) first determine whether or not the net obtained by cutting the two nets is strongly normalizable 2) then (in case it is strongly normalizable) compute the maximal length of the reduction sequences starting from that net.

Relational semantics for linear logic is a form of non-idempotent intersection type system, from which several informations on the execution of a proof-structure can be recovered. An element of the relational interpretation of a proof-structure R with conclusion $\Gamma$ acts thus as a type (refining $\Gamma$) having R as an inhabitant. We are interested in the following type-checking question: given a proof-structure R, a list of formulae $\Gamma$, and a point x in the relational interpretation of $\Gamma$, is x in the interpretation of R? This question is decidable. We present here an algorithm that decides it in time linear in the size of R, if R is a proof-structure in the multiplicative fragment of linear logic. This algorithm can be extended to larger fragments of multiplicative-exponential linear logic containing $\lambda$-calculus.

International audience; It is possible to define a typing system for Multiplicative Exponential Linear Logic (MELL): in such a system, typing judgments are of the form ⊢ R : x : Γ, where R is a MELL proof-structure, Γ is the list of types of the conclusions of R, and x an element of the relational interpretation of Γ, meaning that x is an element of the relational interpretation of R (of type Γ).As relational semantics can be used to infer execution properties of the proof-structure, these judgment can be considered as forms of quantitative typing.We provide an abstract machine that decides, if R satisfies a geometric condition, whether the judgment ⊢ R : x : Γ is valid. Also, the machine halts in bilinear time in the sizes of R and x.

International audience

International audience; We show that: (1) the Taylor expansion of a cut-free MELL proof-structure R with atomic axioms is the (most informative part of the) relational semantics of R; (2) every (connected) MELL proof-net is uniquely determined by the element of order 2 of its Taylor expansion; (3) the relational semantics is injective for (connected) MELL proof-nets.

Proponiamo una rassegna di alcuni aspetti degli sviluppi recenti della teoria della dimostrazione e dei suoi legami con l'informatica.

We give a semantic account of the execution time (i.e. the number of cut-elimination steps leading to the normal form) of an untyped MELL (proof-)net. We first prove that: 1) a net is head-normalizable (i.e. normalizable at depth 0) if and only if its interpretation in the multiset based relational semantics is not empty and 2) a net is normalizable if and only if its exhaustive interpretation (a suitable restriction of its interpretation) is not empty. We then define a size on every experiment of a net, and we precisely relate the number of cut-elimination steps of every stratified reduction sequence to the size of a particular experiment. Finally, we give a semantic measure of execution time: we prove that we can compute the number of cut-elimination steps leading to a cut free normal form of the net obtained by connecting two cut free nets by means of a cut link, from the interpretations of the two cut free nets. These results are inspired by similar ones obtained by the first author for the (untyped) lambda-calculus.

This is a preliminary report, in which we prove the so-called standardization lemma for the additives, in presence of a generalization (the natural one) of the commutative elementary reduction step. The technical tools developped to achieve this result suggest some remarks on the non confluence of full linear logic.

A Multiplicative-Exponential Linear Logic (MELL) proof-structure can be expanded into a set of resource proof-structures: its Taylor expansion. We introduce a new criterion characterizing (and deciding in the finite case) those sets of resource proof-structures that are part of the Taylor expansion of some MELL proof-structure, through a rewriting system acting both on resource and MELL proof-structures. We also prove semi-decidability of the type inhabitation problem for cut-free MELL proof-structures.

Process calculi based on logic, such as πDILL and CP, provide a foundation for deadlock-free concurrent programming. However, in previous work, there is a mismatch between the rules for constructing proofs and the term constructors of the π-calculus: the fundamental operator for parallel composition does not correspond to any rule of linear logic. Kokke et al. [12] introduced Hypersequent Classical Processes (HCP), which addresses this mismatch using hypersequents (collections of sequents) to register parallelism in the typing judgements. However, the step from CP to HCP is a big one. As of yet, HCP does not have reduction semantics, and the addition of delayed actions means that CP processes interpreted as HCP processes do not behave as they would in CP. We introduce HCP − , a variant of HCP with reduction semantics and without delayed actions. We prove progress, preservation, and termination, and show that HCP − supports the same communication protocols as CP.

International audience; We introduce a new graphical representation for multiplica-tive and exponential linear logic proof-structures, based only on standard labelled oriented graphs and standard notions of graph theory. The inductive structure of boxes is handled by means of a box-tree. Our proof-structures are canonical and allows for an elegant definition of their Taylor expansion by means of pullbacks.

L'opera si propone come testo di riferimento per acquisire una solida preparazione specialistica nella Logica, presentando in maniera rigorosa ed innovativa argomenti tradizionalmente affrontati nei corsi universitari di secondo livello. Questo secondo volume, che completa l'opera, presenta le basi della teoria della ricorsività, l'aritmetica di Peano ed i teoremi di incompletezza, gli assiomi della teoria assiomatica degli insiemi di Zermelo-Fraenkel e la teoria degli ordinali e dei cardinali che ne deriva.