Johns Hopkins Category Theory Seminar

Alternate Thursdays
pretalk 4pm, talk 5:30pm

Krieger 413

Fall 2017 Schedule (approximately alternate Thursdays; pretalk 4pm, talk 5:30pm; Krieger 413): Title: Basic concepts of enriched category theory
     The plan is to give a leisurely introduction to the basic concepts of enriched category, in which the collection of morphisms between each fixed pair of objects is itself an object of another category, which will also function as a review of unenriched category theory, in which this collection of morphisms is a mere set. In particular, we will define enriched categories, functors, natural transformations, adjunctions, and at least state the enriched Yoneda lemma.
     At the break we will hold the organizational meeting for the seminar to assign speakers and topics for the rest of the semester. Title:Yoneda, rich and poor
     Our goal is to discuss the Yoneda lemma, first in the setting of enriched category theory, and then in the specific setting of preorders. The Yoneda lemma talks about the embedding of a (V-)category in the category of (V-valued) presheaves on it. We first recall how any closed symmetric monoidal category is enriched over itself, and we will discuss enriched notions of natural transformation. This will allow us to state the enriched Yoneda lemma, and to understand why it is true. In the poor (but, I will argue, not quite bankrupt) setting where V is the preorder 2 = {0 → 1}, a V-category is of course just a preorder, and Yoneda’s embedding is an example of an ideal completion of a preorder. Research into completions of preorders has developed mostly separately from enriched category theory. We will mention a few constructions of other preorder completions, and discuss if, and how, they relate to the general enriched setting.
Title:All good things must come to an end
     In this talk we will begin by examining the calculus of (co)wedges through the careful distillation of select properties of the usual adjunction Set(AxB,C) = Set(A,[B,C]); the further motivating example of identity arrows in a category will lead us to consider the general notion of (co)ends. From here we will move to discuss several exciting and important theorems about ends, such as (but not limited to): (co)limits as (co)ends as (co)limits, Fubini's theorem, the (co)Yoneda lemma(s), and Kan extensions as ends. As we go we will examine as many examples as time and interest allow for, all with an eye to the connections with enriched category theory.
Title: Weighted (co)limits in the unenriched setting
     We'll start by reviewing the classical notion of (co)limit and reinterpreting it in terms of cones. We'll then generalize these notions by adding weights. We'll see the Grothendieck construction of a weighted (co)limit. Finally we'll rewrite these in terms of (co)ends so that they can be generalized to an enriched setting. The reference is Ch. 7.1 and 7.2 from Categorical Homotopy Theory by Emily Riehl.
Title: Enriched weighted colimits
     This week we will enrich what we've learned about ends and colimits over the last two weeks. The first half of the lecture will develop the theory of enriched weighted limits, conical limits, and enriched ends. Special emphasis will be placed on the power of representability, and how these constructions can actually be computed in a V-(co)complete setting. The second half will focus on specific examples and applications of weighted colimits primarily in topology, though participants are encouraged to bring their favorite examples, time permitted. Title: Yoga of Four Operations
     Push, pull, tensor, and hom; these are the four operations of the calculus of bimodules. In this talk, we'll take an "equipment-theoretic" approach to enriched category theory, and express some of the techniques we have learned so far in terms of bimodules between enriched categories, their tensors, and their homs. In particular, we will define Kan extensions and weighted (co)limits and prove a few of their elementary properties. Finally, we will see how these notions play out in the (relatively) simple equipment of sets, functions, and relations. Hopefully, everyone will leave this talk well equipped for the wonderful world of enriched categories. Title:Basics on 2-categories
     We will give an elementary description of double category and 2-category. We will learn how to paste 2-cells. And we will enrich our definitions in 1-categories to 2-categories. Title:Introduction to model categories
     Model categories are good places to do homotopy theory. In such a category, one can indeed define a good notion of homotopy between maps. For example, the category of topological spaces can be endowed with a model structure in which the notion of homotopy between continuous maps is the usual one. In this talk, we will first see how model categories and (right) homotopies are defined, and how their associated homotopy category is constructed. Then we will introduce the injective and projective model structures on a category of diagrams in a “good” model category, which allows us for example to define homotopy colimits (or limits) as the left (or right) adjoint of the homotopy diagonal functor. Finally, we will see that these model structures also apply on enriched diagrams in some closed symmetric monoidal model categories. Title:2-cats with PIEs
     We will specialise the theory of weighted (co)limits to the case V=Cat. By means of several examples, we will learn to spell out the universal property of weighted (co)limits (in both its 1- and 2-dimensional aspect), and construct weighted (co)limits in the 2-category of small categories. We will also give conditions for a 2-category to admit certain classes of weighted (co)limits.
Fall 2016 Schedule (Occasional Mondays; pretalk 3pm, talk 4:30pm; Maryland 114): Title: Factorisation systems in category theory
     An (orthogonal) factorisation system on a category C consists of two classes (E,M) of morphisms in C, subject to an "orthogonality" axiom and both closed under composition and containing the isomorphisms, such that every morphism f in C factorises as f = me, with e in E and m in M. The axioms imply that the (E,M)-factorisations of a given morphism are unique up to unique isomorphism. As in the example of the image factorisation system (surjective, injective) on the category of sets, one can generally see a factorisation system on a category as providing a notion of image to its morphisms.
     Following an introduction to the basic properties and examples of factorisation systems, this talk will survey the role of factorisation systems in such diverse topics of category theory as reflective subcategories, notions of epimorphism, regular categories and (bi)categories of relations, and constructions of associated sheaves, as dictated by time and interest.
Title: Simplicial objects
     Simplicial objects provide a combinatorial model for doing homotopy theory, that generalize topological spaces and chain complexes simultaneously. In this talk we'll try and understand a multitude of examples of simplicial objects. We'll describe the homotopy groups of Kan complexes (fibrant objects in Quillen's model category). We'll state the connection between simplicial categories and other categories via dold-kan and geometric realization functors.
Title: An introduction to the Galois theory of Grothendieck
     In SGA1, Grothendieck re-imagines Galois theory in terms of an axiomatic characterisation of categories of group actions. This approach leads to generalisations of Galois theory which applies to infinite field extensions and to categories of commutative algebras. Maybe more surprisingly he is able to demonstrate that, in this framework, Galois theory and the theory of covering spaces / the fundamental group become examples of the same categorical formalism. Ultimately, Grothendieck's insight gains it most abstract and all encompassing form in the work of Joyal and Tierney, in which they demonstrate that any Grothendieck topos, that is to say any category of of generalised sheaves, is equivalent to a category of continuous actions of some localic (spatial) groupoid. We might call this the Galois groupoid of the topos.
     In this talk I intend to provide an elementary introduction to Grothendieck's Galois theory, proving as many of his core results as I can in the time available. We will examine both finite and pro-finite variants, which will enable us to discuss the Galois theory of (infinite) algebraic closures and the theory of covering spaces of spaces that lack a universal covering space. I also hope to briefly touch upon the primary themes and motivations that arise in Joyal and Tierney's work. My plan is to rely on few categorical preliminaries, and to explain those that I do need as we go along. Title: A monad is just ... (with an eye to universal algebra)
      In this talk we aim to explore some of the elementary results in the theory of monads. We will begin by examining how monads might naturally arise from notions in monoidal categories and attempt to understand monads and their modules (specifically algebras) in this light — finding, as we will, the generation of monads from monoids to be unsatisfactory in that the inverse problem does not admit an immediately natural and adequate resolution. With that source of monads exhausted we will turn to the next and perhaps most fruitful approach, viz., the generation of monads from adjunctions.
     Once some basic results have been established we will address the inverse problem again and find here the famous results of Kleisli and Eilenberg-Moore: every monad arises from an adjunction in two canonical and somehow universal ways. We will explore the Kleisli category and Eielenberg-Moore categories for some common monads in order to impress the niceties of the latter, the category of algebras. In particular we will interest ourselves with the observation that the construction of Eilenberg-Moore inherits all limits and suitable colimits from the base category and so prompt the answering of a very practical question: how might we tell, for a given “algebraic” system (groups, monoids, categories ...), whether and indeed which limits and colimits exist? This will lead us to the notion of monadicity and the theorem of Beck.
     Time permitting we will consider the idea of a distributive law so that we may more clearly describe the situation of one monad acting over another — the generalisation of the case of unital rings in which the multiplicative monoid and additive group interact suitably.
     Only a knowledge of categories, functors, natural transformations, limits and adjoints will be assumed.

Contact Emily Riehl.