arXiv subjects
Publications and source records attributed to Krzysztof Worytkiewicz.
We continue our work on implicative assemblies by investigating under which circumstances a subset $M \subseteq \mathscr{S}$ gives rise to a lex full subcategory $\mathbf{Asm}_M$ of the quasitopos $\mathbf{Asm}_{\mathcal{A}}$ of all assemblies such that $(\mathbf{Asm}_M)_{reg/lex} \simeq \mathbf{Asm}_{\mathcal{A}}$. We establish a characterisation. Furthermore, this latter is relevant to the study of $\mathbf{Asm}_M$'s ex/lex-completion.
Implicative algebras, recently discovered by Miquel, are combinatorial structures unifying classical and intuitionistic realizability as well as forcing. In this paper we introduce implicative assemblies as sets valued in the separator of an underlying implicative algebra. Given a fixed implicative algebra A, implicative assemblies over A organise themselves in a category Asm with tracked set-theoretical functions as morphisms. We show that Asm is a quasitopos with NNO.
We study formal counterparts of Hurewicz fibrations and related topological notions in elementary toposes with NNO. The constructions are based on a specific notion of interval and lead to a structure of category of fibrant objects on toposes equipped with such a datum. We get in fact slightly more as the building blocks are derived from a weak factorisation system.
This paper introduces a novel approach to the axiomatic theory of quadratic forms. We work internally in a category of certain partially ordered sets, subject to additional conditions which amount to a strong form of local presentability. We call such partial orders presentable. It turns out that the classical notion of the Witt ring of symmetric bilinear forms over a field makes sense in the context of quadratically presentable fields, that is fields equipped with a presentable partial order inequationaly compatible with the algebraic operations. As an application, we show that Witt rings of symmetric bilinear forms over fields, of both characteristic 2 and not 2, are isomorphic to Witt rings of suitably built quadratically presentable fields, which therefore provide a uniform construction of Witt rings for all characteristics.
We study the homotopy theory of locally ordered spaces, that is manifolds with boundary whose charts are partially ordered in a compatible way. Their category is not particularly well-behaved with respect to colimits. However, this category turns out to be a certain full subcategory of a topos of sheaves over a simpler site. A precise characterisation of this subcategory is provided. The ambient topos makes available some general homotopical machinery.
We establish a model structure on the category of strict omega-categories. The constructions leading to the model structure in question are expressed entirely within the scope of omega-categories, building on a set of generating cofibrations and a class of weak equivalences as basic items. All object are fibrant while cofibrant objects are exactly the free ones. Our model structure transfers to n-categories along right-adjoints, for each n, thus recovering the known cases n = 1 and n = 2.
In this paper we show that classical notions from automata theory such as simulation and bisimulation can be lifted to the context of enriched categories. The usual properties of bisimulation are nearly all preserved in this new context. The class of enriched functors that correspond to functionnal bisimulations surjective on objects is investigated and appears "nearly" open in the sense of Joyal and Moerdijk. Seeing the change of base techniques as a convenient means to define process refinement/abstractions, we give sufficient conditions for the change of base categories to preserve bisimularity. We apply these concepts to Betti's generalized automata, categorical transition systems, and other exotic categories.
Locally partial-ordered spaces (local po-spaces) have been used to model concurrent systems. We provide equivalences for these spaces by constructing a model category containing the category of local po-spaces. We show the category of simplicial presheaves on local po-spaces can be given Jardine's model structure, in which we identify the weak equivalences between local po-spaces. In the process we give an equivalence between the category of sheaves on a local po-space and the category of etale bundles over a local po-space. Finally we describe a localization that should provide a good framework for studying concurrent systems.
We introduce a notion of synchronization for higher-dimensional automata, based on coskeletons of cubical sets. Categorification transports this notion to the setting of categorical transition systems. We apply the results to study the semantics of an imperative programming language with message-passing.