Coequalisers of formal topology

We give a predicative construction of quotients of formal topologies. Along with earlier results on the match up between of continuous functions on real numbers (in the sense of Bishop\u27s constructive mathematics) and approximable mappings on the formal space of reals, we argue that formal topology gives an adequate foundation for constructive algebraic topology, also in the predicative sense. Predicativity is of essence when formalising the subject in logical frameworks based on Martin-Löf type theories

Lex colimits

Many kinds of categorical structure require the existence of finite limits, of colimits of some specified type, and of "exactness" conditions between the finite limits and the specified colimits. Some examples are the notions of regular, or Barr-exact, or lextensive, or coherent, or adhesive category. We introduce a general notion of exactness, of which each of the structures listed above, and others besides, are particular instances. The notion can be understood as a form of cocompleteness "in the lex world" -- more precisely, in the 2-category of finitely complete categories and finite-limit preserving functors.Comment: 38 pages; v2: final journal version, various minor changes and new Section 5.9 dealing with filtered colimit

GG-typical Witt vectors with coefficients and the norm

For a profinite group $G$ we describe an abelian group $W_G(R; M)$ of $G$-typical Witt vectors with coefficients in an $R$-module $M$ (where $R$ is a commutative ring). This simultaneously generalises the ring $W_G(R)$ of Dress and Siebeneicher and the Witt vectors with coefficients $W(R; M)$ of Dotto, Krause, Nikolaus and Patchkoria, both of which extend the usual Witt vectors of a ring. We use this new variant of Witt vectors to give a purely algebraic description of the zeroth equivariant stable homotopy groups of the Hill-Hopkins-Ravenel norm $N_{\{e\}}^G(X)$ of a connective spectrum $X$, for any finite group $G$. Our construction is reasonably analogous to the constructions of previous variants of Witt vectors, and as such is amenable to fairly explicit concrete computations.Comment: 83 page

Categorical generalisations of quantum double models

We show that every involutive Hopf monoid in a complete and finitely cocomplete symmetric monoidal category gives rise to invariants of oriented surfaces defined in terms of ribbon graphs. For every ribbon graph this yields an object in the category, defined up to isomorphism, that depends only on the homeomorphism class of the associated surface. This object is constructed via (co)equalisers and images and equipped with a mapping class group action. It can be viewed as a categorical generalisation of the ground state of Kitaev's quantum double model or of a representation variety for a surface. We apply the construction to group objects in cartesian monoidal categories, in particular to simplicial groups as group objects in SSet and to crossed modules as group objects in Cat. The former yields a simplicial set consisting of representation varieties, the latter a groupoid whose sets of objects and morphisms are obtained from representation varieties.Comment: 46 page