arXiv subjects

Seerp Roald Koudenburg

Publications and source records attributed to Seerp Roald Koudenburg.

9 recordsLinked to original sources

arXiv2026

Virtual double categories of split two-sided 2-fibrations

This paper introduces and studies split two-sided 2-fibrations and locally discrete split two-sided 2-fibrations, using a formal categorical approach. We generalise Street's notion of split two-sided fibration internal to a 2-category to one internal to a sesquicategory. Given a sesquicategory we construct a virtual double category whose horizontal (loose) morphisms are its internal split two-sided fibrations. Specialising to the sesquicategory of lax natural transformations we obtain the virtual double category of split two-sided 2-fibrations, which we study in detail. We then restrict to the sub-virtual double category of locally discrete split two-sided 2-fibrations and show that therein the usual Yoneda 2-functors satisfy a double-categorical formal notion of Yoneda morphism, which formally captures universal properties similar to those satisfied by the morphisms comprising a Yoneda structure on a 2-category. As a consequence we obtain a 'two-sided Grothendieck correspondence' of locally discrete split two-sided 2-fibrations $A \nrightarrow B$ and 2-functors $B \to Cat^{A^{op}}$. Restricting to $A = 1$, the terminal 2-category, we improve Buckley and Lambert's 'Grothendieck correspondence' for locally discrete split op-2-fibrations by extending the sense in which it is functorial.

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arXiv2025

Augmented virtual double categories

In this article the notion of virtual double category (also known as fc-multicategory) is extended as follows. While cells in a virtual double category classically have a horizontal multi-source and single horizontal target, the notion of augmented virtual double category introduced here extends the latter notion by including cells with empty horizontal target as well. Any augmented virtual double category comes with a built-in notion of "locally small object" and we describe advantages of using augmented virtual double categories as a setting for formal category rather than 2-categories, which are classically equipped with a notion of "admissible object" by means of a yoneda structure in the sense of Street and Walters. An object is locally small precisely if it admits a horizontal unit, and we show that the notions of augmented virtual double category and virtual double category coincide in the presence of all horizontal units. Without assuming the existence of horizontal units we show that most of the basic theory for virtual double categories, such as that of restriction and composition of horizontal morphisms, extends to augmented virtual double categories. We introduce and study in augmented virtual double categories the notion of "pointwise" composition of horizontal morphisms, which formalises the classical composition of profunctors given by the coend formula.

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arXiv2024

Formal category theory in augmented virtual double categories

In this article we develop formal category theory within augmented virtual double categories. Notably we formalise the classical notions of Kan extension, Yoneda embedding $\text y_A\colon A \to \hat A$, exact square, total category and 'small' cocompletion; the latter in an appropriate sense. Throughout we compare our formalisations to their corresponding $2$-categorical counterparts. Our approach has several advantages. For instance, the structure of augmented virtual double categories naturally allows us to isolate conditions that ensure small cocompleteness of formal presheaf objects $\hat A$. Given a monoidal augmented virtual double category $\mathcal K$ with a Yoneda embedding $\text y_I \colon I \to \hat I$ for its monoidal unit $I$ we prove that, for any 'unital' object $A$ in $\mathcal K$ that has a 'horizontal dual' $A^\circ$, the Yoneda embedding $\text y_A \colon A \to \hat A$ exists if and only if the 'inner hom' $[A^\circ, \hat I]$ exists. This result is a special case of a more general result that, given a functor $F\colon \mathcal K \to \mathcal L$ of augmented virtual double categories, allows a Yoneda embedding in $\mathcal L$ to be "lifted", along a pair of 'universal morphisms' in $\mathcal L$, to a Yoneda embedding in $\mathcal K$.

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arXiv2022

A double-dimensional approach to formal category theory

Whereas formal category theory is classically considered within a $2$-category, in this paper a double-dimensional approach is taken. More precisely we develop such theory within the setting of augmented virtual double categories, a notion extending that of virtual double category by adding cells with nullary target. [...] After this the notion of `weak' Kan extension within an augmented virtual double category is considered, together with three strengthenings. [...] The notion of yoneda embedding is then considered in an augmented virtual double category, and compared to that of a good yoneda structure on a $2$-category; the latter in the sense of Street-Walters and Weber. Conditions are given ensuring that a yoneda embedding $y \colon A \to \hat A$ defines $\hat A$ as the free small cocompletion of $A$, in a suitable sense. In the second half we consider formal category theory in the presence of algebraic structures. In detail: to a monad $T$ on an augmented virtual double category $\mathcal K$ several augmented virtual double categories $T\text-\mathsf{Alg}_{(v, w)}$ of $T$-algebras are associated, [...]. This is followed by the study of the creation of, amongst others, left Kan extensions by the forgetful functors $T\text-\mathsf{Alg}_{(v, w)} \to \mathcal K$. The main motivation of this paper is the description of conditions ensuring that yoneda embeddings in $\mathcal K$ lift along these forgetful functors, as well as ensuring that such lifted algebraic yoneda embeddings again define free small cocompletions, now in $T\text-\mathsf{Alg}_{(v, w)}$. As a first example we apply the previous to monoidal structures on categories, hence recovering Day convolution of presheaves and Im-Kelly's result on free monoidal cocompletion, as well as obtaining a "monoidal Yoneda lemma".

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arXiv2017

A categorical approach to the maximum theorem

Berge's maximum theorem gives conditions ensuring the continuity of an optimised function as a parameter changes. In this paper we state and prove the maximum theorem in terms of the theory of monoidal topology and the theory of double categories. This approach allows us to generalise (the main assertion of) the maximum theorem, which is classically stated for topological spaces, to pseudotopological spaces and pretopological spaces, as well as to closure spaces, approach spaces and probabilistic approach spaces, amongst others. As a part of this we prove a generalisation of the extreme value theorem.

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arXiv2015

Algebraic Kan extensions in double categories

We study Kan extensions in three weakenings of the Eilenberg-Moore double category associated to a double monad, that was introduced by Grandis and Paré. To be precise, given a normal oplax double monad $T$ on a double category $\mathcal K$, we consider the double categories consisting of pseudo $T$-algebras, `weak' vertical $T$-morphisms, horizontal $T$-morphisms and $T$-cells, where `weak' means either `lax', `colax' or `pseudo'. Denoting these double categories by $\mathsf{Alg}_w T$, where w = l, c or ps accordingly, our main result gives, in each of these cases, conditions ensuring that (pointwise) Kan extensions can be lifted along the forgetful double functor $\mathsf{Alg}_w T \to \mathcal K$. As an application we recover and generalise a result by Getzler, on the lifting of pointwise left Kan extensions along symmetric monoidal enriched functors. As an application of Getzler's result we prove, in suitable symmetric monoidal categories, the existence of bicommutative Hopf monoids that are freely generated by cocommutative comonoids.

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arXiv2014

Left Kan extensions that are algebraic over colax-idempotent 2-monads

Using the language of double categories we generalise a classical result on finite-product-preserving left Kan extensions, by Adámek and Rosický, to one on left Kan extensions that preserve algebraic structures defined by `suitable' colax-idempotent 2-monads, as well as obtain two related results. To be precise, by `suitable' 2-monads here we mean ones that extend to normal lax double monads. In an appendix we consider induced algebra structures on presheaf objects.

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arXiv2014

On pointwise Kan extensions in double categories

In this paper we consider a notion of pointwise Kan extension in double categories that naturally generalises Dubuc's notion of pointwise Kan extension along enriched functors. We show that, when considered in equipments that admit opcartesian tabulations, it generalises Street's notion of pointwise Kan extension in 2-categories.

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arXiv2013

Algebraic weighted colimits

In this thesis weighted colimits in 2-categories equipped with promorphisms are studied. Such colimits include most universal constructions with counits, like ordinary colimits in categories, weighted colimits in enriched categories, and left Kan extensions. In the first chapter we recall the notion of 2-categories equipped with promorphisms (also simply called equipments), that provide a coherent way of adding bimodule-like morphisms to a 2-category. In the second chapter we recall two ways of defining weighted colimits in equipments. Most important to us is their original definition, introduced by Wood, in equipments that are endowed with a closed structure. The second notion, of what we call pointwise weighted colimits, was introduced by Grandis and Paré. It requires no extra structure and generalises Street's notion of pointwise left Kan extensions in 2-categories. The main result of the second chapter gives a condition, on closed equipments, under which these two notions coincide. In the third chapter we consider monads on equipments. The main idea of this thesis, given in the fourth chapter, generalises the notions of lax and colax morphisms, of algebras over a 2-monad, to notions of lax and colax promorphisms, of algebras over a monad on an equipment. One of these, that of right colax promorphisms, is well suited to the construction of weighted colimits. In particular, given a monad T on an equipment K, we will show that T-algebras, colax T-morphisms and right colax T-promorphisms form a double category T-rcProm. Although weaker than equipments, double categories still allow definition of weighted colimits, and our main result states that the forgetful functor T-rcProm -> K lifts all weighted colimits whenever K is closed, under some mild conditions on T.

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