The paper contains a relatively non-technical summary of some recent work by the author and Jeremy Butterfield. The goal is to find a way of assigning meaningful truth values to propositions in quantum theory: something that is not possible in the normal, instrumentalist interpretation. The key mathematical tool is presheaf theory where, multi-valued, contextual truth values arise naturally. We...

In previous work with Pous, we defined a semantics for CCS which may both be viewed as an innocent presheaf semantics and as a concurrent game semantics. It is here proved that a behavioural equivalence induced by this semantics on CCS processes is fully abstract for fair testing equivalence. The proof relies on a new algebraic notion called playground, which represents the ‘rule of the game’. ...

Abstract syntax with variable binding is known to be characterised as an initial algebra in a presheaf category. This paper extends it to the case of polymorphic typed abstract syntax with binding. We consider two variations, secondorder and higher-order polymorphic syntax. The central idea is to apply Fiore’s initial algebra characterisation of typed abstract syntax with binding repeatedly, i....

We study generalised polynomial functors between presheaf categories, developing their mathematical theory together with computational applications. The main theoretical contribution is the introduction of discrete generalised polynomial functors, a class that lies in between the classes of cocontinuous and finitary functors, and is closed under composition, sums, finite products, and different...

For a small quantaloid Q we consider four fundamental 2-monads T on Q-Cat, given by the presheaf 2-monad P and the copresheaf 2-monad P†, as well as their two composite 2-monads, and establish that they all laxly distribute over P. These four 2-monads therefore admit lax extensions to the category Q-Dist of Q-categories and their distributors. We characterize the corresponding (T,Q)-categories ...

A conjecture of F.Morel states that the motivic group π0,0(k) of a perfect field k coincides with the Grothendieck-Witt group GW (k) of quadratic forms over k provided that char(k) 6= 2. Morel’s proof of the conjecture requires the the following result: the Nisnevich sheaf WNis associated with the presheaf X 7→ W (X) is homotopy invariant and all its Nisnevich cohomology are homotopy invariant ...

We prove a multiplicative version of the equivariant Barratt-Priddy-Quillen theorem, starting from additive proven in [13]. The proof uses elaboration an infinite loop space machine that manufactures orthogonal G-spectra symmetric monoidal G-categories. new produces highly structured associative ring and module appropriate input. It relies on operadic multicategories are considerable independen...

We define a semantics for Milner’s pi-calculus, with three main novelties. First, it provides a fully-abstract model for fair testing equivalence, whereas previous semantics covered variants of bisimilarity and the may and must testing equivalences. Second, it is based on reduction semantics, whereas previous semantics were based on labelled transition systems. Finally, it has a strong game sem...

The notion of Σ-monoids is proposed by Fiore, Plotkin and Turi, to give abstract algebraic model of languages with variable binding and substitutions. In this paper, we give a free construction of Σ-monoids. The free Σ-monoid over a given presheaf serves a well-structured term language involving binding and substitutions. Moreover, the free Σ-monoid naturally contains interesting syntactic obje...

We develop a 2-categorical theory for recursively defined domains. In particular, we generalise the traditional approach based on ordertheoretic structures to category-theoretic ones. A motivation for this development is the need of a domain theory for concurrency, with an account of bisimulation. Indeed, the leading examples throughout the paper are provided by recursively defined presheaf mod...