PARTIAL TOPOSES 1. Partial Toposes
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A Convenient Category for Higher-Order Probability Theory
A Convenient Category for Higher-Order Probability Theory Chris Heunen Ohad Kammar Sam Staton Hongseok Yang University of Edinburgh, UK University of Oxford, UK University of Oxford, UK University of Oxford, UK Abstract—Higher-order probabilistic programming languages 1 (defquery Bayesian-linear-regression allow programmers to write sophisticated models in machine 2 let let sample normal learning and statistics in a succinct and structured way, but step ( [f ( [s ( ( 0.0 3.0)) 3 sample normal outside the standard measure-theoretic formalization of proba- b ( ( 0.0 3.0))] 4 fn + * bility theory. Programs may use both higher-order functions and ( [x] ( ( s x) b)))] continuous distributions, or even define a probability distribution 5 (observe (normal (f 1.0) 0.5) 2.5) on functions. But standard probability theory does not handle 6 (observe (normal (f 2.0) 0.5) 3.8) higher-order functions well: the category of measurable spaces 7 (observe (normal (f 3.0) 0.5) 4.5) is not cartesian closed. 8 (observe (normal (f 4.0) 0.5) 6.2) Here we introduce quasi-Borel spaces. We show that these 9 (observe (normal (f 5.0) 0.5) 8.0) spaces: form a new formalization of probability theory replacing 10 (predict :f f))) measurable spaces; form a cartesian closed category and so support higher-order functions; form a well-pointed category and so support good proof principles for equational reasoning; and support continuous probability distributions. We demonstrate the use of quasi-Borel spaces for higher-order functions and proba- bility by: showing that a well-known construction of probability theory involving random functions gains a cleaner expression; and generalizing de Finetti’s theorem, that is a crucial theorem in probability theory, to quasi-Borel spaces. -
Limits Commutative Algebra May 11 2020 1. Direct Limits Definition 1
Limits Commutative Algebra May 11 2020 1. Direct Limits Definition 1: A directed set I is a set with a partial order ≤ such that for every i; j 2 I there is k 2 I such that i ≤ k and j ≤ k. Let R be a ring. A directed system of R-modules indexed by I is a collection of R modules fMi j i 2 Ig with a R module homomorphisms µi;j : Mi ! Mj for each pair i; j 2 I where i ≤ j, such that (i) for any i 2 I, µi;i = IdMi and (ii) for any i ≤ j ≤ k in I, µi;j ◦ µj;k = µi;k. We shall denote a directed system by a tuple (Mi; µi;j). The direct limit of a directed system is defined using a universal property. It exists and is unique up to a unique isomorphism. Theorem 2 (Direct limits). Let fMi j i 2 Ig be a directed system of R modules then there exists an R module M with the following properties: (i) There are R module homomorphisms µi : Mi ! M for each i 2 I, satisfying µi = µj ◦ µi;j whenever i < j. (ii) If there is an R module N such that there are R module homomorphisms νi : Mi ! N for each i and νi = νj ◦µi;j whenever i < j; then there exists a unique R module homomorphism ν : M ! N, such that νi = ν ◦ µi. The module M is unique in the sense that if there is any other R module M 0 satisfying properties (i) and (ii) then there is a unique R module isomorphism µ0 : M ! M 0. -
A Few Points in Topos Theory
A few points in topos theory Sam Zoghaib∗ Abstract This paper deals with two problems in topos theory; the construction of finite pseudo-limits and pseudo-colimits in appropriate sub-2-categories of the 2-category of toposes, and the definition and construction of the fundamental groupoid of a topos, in the context of the Galois theory of coverings; we will take results on the fundamental group of étale coverings in [1] as a starting example for the latter. We work in the more general context of bounded toposes over Set (instead of starting with an effec- tive descent morphism of schemes). Questions regarding the existence of limits and colimits of diagram of toposes arise while studying this prob- lem, but their general relevance makes it worth to study them separately. We expose mainly known constructions, but give some new insight on the assumptions and work out an explicit description of a functor in a coequalizer diagram which was as far as the author is aware unknown, which we believe can be generalised. This is essentially an overview of study and research conducted at dpmms, University of Cambridge, Great Britain, between March and Au- gust 2006, under the supervision of Martin Hyland. Contents 1 Introduction 2 2 General knowledge 3 3 On (co)limits of toposes 6 3.1 The construction of finite limits in BTop/S ............ 7 3.2 The construction of finite colimits in BTop/S ........... 9 4 The fundamental groupoid of a topos 12 4.1 The fundamental group of an atomic topos with a point . 13 4.2 The fundamental groupoid of an unpointed locally connected topos 15 5 Conclusion and future work 17 References 17 ∗e-mail: [email protected] 1 1 Introduction Toposes were first conceived ([2]) as kinds of “generalised spaces” which could serve as frameworks for cohomology theories; that is, mapping topological or geometrical invariants with an algebraic structure to topological spaces. -
Basic Category Theory and Topos Theory
Basic Category Theory and Topos Theory Jaap van Oosten Jaap van Oosten Department of Mathematics Utrecht University The Netherlands Revised, February 2016 Contents 1 Categories and Functors 1 1.1 Definitions and examples . 1 1.2 Some special objects and arrows . 5 2 Natural transformations 8 2.1 The Yoneda lemma . 8 2.2 Examples of natural transformations . 11 2.3 Equivalence of categories; an example . 13 3 (Co)cones and (Co)limits 16 3.1 Limits . 16 3.2 Limits by products and equalizers . 23 3.3 Complete Categories . 24 3.4 Colimits . 25 4 A little piece of categorical logic 28 4.1 Regular categories and subobjects . 28 4.2 The logic of regular categories . 34 4.3 The language L(C) and theory T (C) associated to a regular cat- egory C ................................ 39 4.4 The category C(T ) associated to a theory T : Completeness Theorem 41 4.5 Example of a regular category . 44 5 Adjunctions 47 5.1 Adjoint functors . 47 5.2 Expressing (co)completeness by existence of adjoints; preserva- tion of (co)limits by adjoint functors . 52 6 Monads and Algebras 56 6.1 Algebras for a monad . 57 6.2 T -Algebras at least as complete as D . 61 6.3 The Kleisli category of a monad . 62 7 Cartesian closed categories and the λ-calculus 64 7.1 Cartesian closed categories (ccc's); examples and basic facts . 64 7.2 Typed λ-calculus and cartesian closed categories . 68 7.3 Representation of primitive recursive functions in ccc's with nat- ural numbers object . -
A Small Complete Category
Annals of Pure and Applied Logic 40 (1988) 135-165 135 North-Holland A SMALL COMPLETE CATEGORY J.M.E. HYLAND Department of Pure Mathematics and Mathematical Statktics, 16 Mill Lane, Cambridge CB2 ISB, England Communicated by D. van Dalen Received 14 October 1987 0. Introduction This paper is concerned with a remarkable fact. The effective topos contains a small complete subcategory, essentially the familiar category of partial equiv- alence realtions. This is in contrast to the category of sets (indeed to all Grothendieck toposes) where any small complete category is equivalent to a (complete) poset. Note at once that the phrase ‘a small complete subcategory of a topos’ is misleading. It is not the subcategory but the internal (small) category which matters. Indeed for any ordinary subcategory of a topos there may be a number of internal categories with global sections equivalent to the given subcategory. The appropriate notion of subcategory is an indexed (or better fibred) one, see 0.1. Another point that needs attention is the definition of completeness (see 0.2). In my talk at the Church’s Thesis meeting, and in the first draft of this paper, I claimed too strong a form of completeness for the internal category. (The elementary oversight is described in 2.7.) Fortunately during the writing of [13] my collaborators Edmund Robinson and Giuseppe Rosolini noticed the mistake. Again one needs to pay careful attention to the ideas of indexed (or fibred) categories. The idea that small (sufficiently) complete categories in toposes might exist, and would provide the right setting in which to discuss models for strong polymorphism (quantification over types), was suggested to me by Eugenio Moggi. -
The Hecke Bicategory
Axioms 2012, 1, 291-323; doi:10.3390/axioms1030291 OPEN ACCESS axioms ISSN 2075-1680 www.mdpi.com/journal/axioms Communication The Hecke Bicategory Alexander E. Hoffnung Department of Mathematics, Temple University, 1805 N. Broad Street, Philadelphia, PA 19122, USA; E-Mail: [email protected]; Tel.: +215-204-7841; Fax: +215-204-6433 Received: 19 July 2012; in revised form: 4 September 2012 / Accepted: 5 September 2012 / Published: 9 October 2012 Abstract: We present an application of the program of groupoidification leading up to a sketch of a categorification of the Hecke algebroid—the category of permutation representations of a finite group. As an immediate consequence, we obtain a categorification of the Hecke algebra. We suggest an explicit connection to new higher isomorphisms arising from incidence geometries, which are solutions of the Zamolodchikov tetrahedron equation. This paper is expository in style and is meant as a companion to Higher Dimensional Algebra VII: Groupoidification and an exploration of structures arising in the work in progress, Higher Dimensional Algebra VIII: The Hecke Bicategory, which introduces the Hecke bicategory in detail. Keywords: Hecke algebras; categorification; groupoidification; Yang–Baxter equations; Zamalodchikov tetrahedron equations; spans; enriched bicategories; buildings; incidence geometries 1. Introduction Categorification is, in part, the attempt to shed new light on familiar mathematical notions by replacing a set-theoretic interpretation with a category-theoretic analogue. Loosely speaking, categorification replaces sets, or more generally n-categories, with categories, or more generally (n + 1)-categories, and functions with functors. By replacing interesting equations by isomorphisms, or more generally equivalences, this process often brings to light a new layer of structure previously hidden from view. -
SHORT INTRODUCTION to ENRICHED CATEGORIES FRANCIS BORCEUX and ISAR STUBBE Département De Mathématique, Université Catholique
SHORT INTRODUCTION TO ENRICHED CATEGORIES FRANCIS BORCEUX and ISAR STUBBE Departement de Mathematique, Universite Catholique de Louvain, 2 Ch. du Cyclotron, B-1348 Louvain-la-Neuve, Belgium. e-mail: [email protected] — [email protected] This text aims to be a short introduction to some of the basic notions in ordinary and enriched category theory. With reasonable detail but always in a compact fashion, we have brought together in the rst part of this paper the denitions and basic properties of such notions as limit and colimit constructions in a category, adjoint functors between categories, equivalences and monads. In the second part we pass on to enriched category theory: it is explained how one can “replace” the category of sets and mappings, which plays a crucial role in ordinary category theory, by a more general symmetric monoidal closed category, and how most results of ordinary category theory can be translated to this more general setting. For a lack of space we had to omit detailed proofs, but instead we have included lots of examples which we hope will be helpful. In any case, the interested reader will nd his way to the references, given at the end of the paper. 1. Ordinary categories When working with vector spaces over a eld K, one proves such theorems as: for all vector spaces there exists a base; every vector space V is canonically included in its bidual V ; every linear map between nite dimensional based vector spaces can be represented as a matrix; and so on. But where do the universal quantiers take their value? What precisely does “canonical” mean? How can we formally “compare” vector spaces with matrices? What is so special about vector spaces that they can be based? An answer to these questions, and many more, can be formulated in a very precise way using the language of category theory. -
The Category of Sheaves Is a Topos Part 2
The category of sheaves is a topos part 2 Patrick Elliott Recall from the last talk that for a small category C, the category PSh(C) of presheaves on C is an elementary topos. Explicitly, PSh(C) has the following structure: • Given two presheaves F and G on C, the exponential GF is the presheaf defined on objects C 2 obC by F G (C) = Hom(hC × F; G); where hC = Hom(−;C) is the representable functor associated to C, and the product × is defined object-wise. • Writing 1 for the constant presheaf of the one object set, the subobject classifier true : 1 ! Ω in PSh(C) is defined on objects by Ω(C) := fS j S is a sieve on C in Cg; and trueC : ∗ ! Ω(C) sends ∗ to the maximal sieve t(C). The goal of this talk is to refine this structure to show that the category Shτ (C) of sheaves on a site (C; τ) is also an elementary topos. To do this we must make use of the sheafification functor defined at the end of the first talk: Theorem 0.1. The inclusion functor i : Shτ (C) ! PSh(C) has a left adjoint a : PSh(C) ! Shτ (C); called sheafification, or the associated sheaf functor. Moreover, this functor commutes with finite limits. Explicitly, a(F) = (F +)+, where + F (C) := colimS2τ(C)Match(S; F); where Match(S; F) is the set of matching families for the cover S of C, and the colimit is taken over all covering sieves of C, ordered by reverse inclusion. -
Notes on Categorical Logic
Notes on Categorical Logic Anand Pillay & Friends Spring 2017 These notes are based on a course given by Anand Pillay in the Spring of 2017 at the University of Notre Dame. The notes were transcribed by Greg Cousins, Tim Campion, L´eoJimenez, Jinhe Ye (Vincent), Kyle Gannon, Rachael Alvir, Rose Weisshaar, Paul McEldowney, Mike Haskel, ADD YOUR NAMES HERE. 1 Contents Introduction . .3 I A Brief Survey of Contemporary Model Theory 4 I.1 Some History . .4 I.2 Model Theory Basics . .4 I.3 Morleyization and the T eq Construction . .8 II Introduction to Category Theory and Toposes 9 II.1 Categories, functors, and natural transformations . .9 II.2 Yoneda's Lemma . 14 II.3 Equivalence of categories . 17 II.4 Product, Pullbacks, Equalizers . 20 IIIMore Advanced Category Theoy and Toposes 29 III.1 Subobject classifiers . 29 III.2 Elementary topos and Heyting algebra . 31 III.3 More on limits . 33 III.4 Elementary Topos . 36 III.5 Grothendieck Topologies and Sheaves . 40 IV Categorical Logic 46 IV.1 Categorical Semantics . 46 IV.2 Geometric Theories . 48 2 Introduction The purpose of this course was to explore connections between contemporary model theory and category theory. By model theory we will mostly mean first order, finitary model theory. Categorical model theory (or, more generally, categorical logic) is a general category-theoretic approach to logic that includes infinitary, intuitionistic, and even multi-valued logics. Say More Later. 3 Chapter I A Brief Survey of Contemporary Model Theory I.1 Some History Up until to the seventies and early eighties, model theory was a very broad subject, including topics such as infinitary logics, generalized quantifiers, and probability logics (which are actually back in fashion today in the form of con- tinuous model theory), and had a very set-theoretic flavour. -
SHEAVES, TOPOSES, LANGUAGES Acceleration Based on A
Chapter 7 Logic of behavior: Sheaves, toposes, and internal languages 7.1 How can we prove our machine is safe? Imagine you are trying to design a system of interacting components. You wouldn’t be doing this if you didn’t have a goal in mind: you want the system to do something, to behave in a certain way. In other words, you want to restrict its possibilities to a smaller set: you want the car to remain on the road, you want the temperature to remain in a particular range, you want the bridge to be safe for trucks to pass. Out of all the possibilities, your system should only permit some. Since your system is made of components that interact in specified ways, the possible behavior of the whole—in any environment—is determined by the possible behaviors of each of its components in their local environments, together with the precise way in which they interact.1 In this chapter, we will discuss a logic wherein one can describe general types of behavior that occur over time, and prove properties of a larger-scale system from the properties and interaction patterns of its components. For example, suppose we want an autonomous vehicle to maintain a distance of some safe R from other objects. To do so, several components must interact: a 2 sensor that approximates the real distance by an internal variable S0, a controller that uses S0 to decide what action A to take, and a motor that moves the vehicle with an 1 The well-known concept of emergence is not about possibilities, it is about prediction. -
On Closed Categories of Functors
ON CLOSED CATEGORIES OF FUNCTORS Brian Day Received November 7, 19~9 The purpose of the present paper is to develop in further detail the remarks, concerning the relationship of Kan functor extensions to closed structures on functor categories, made in "Enriched functor categories" | 1] §9. It is assumed that the reader is familiar with the basic results of closed category theory, including the representation theorem. Apart from some minor changes mentioned below, the terminology and notation employed are those of |i], |3], and |5]. Terminology A closed category V in the sense of Eilenberg and Kelly |B| will be called a normalised closed category, V: V o ÷ En6 being the normalisation. Throughout this paper V is taken to be a fixed normalised symmetric monoidal closed category (Vo, @, I, r, £, a, c, V, |-,-|, p) with V ° admitting all small limits (inverse limits) and colimits (direct limits). It is further supposed that if the limit or colimit in ~o of a functor (with possibly large domain) exists then a definite choice has been made of it. In short, we place on V those hypotheses which both allow it to replace the cartesian closed category of (small) sets Ens as a ground category and are satisfied by most "natural" closed categories. As in [i], an end in B of a V-functor T: A°P@A ÷ B is a Y-natural family mA: K ÷ T(AA) of morphisms in B o with the property that the family B(1,mA): B(BK) ÷ B(B,T(AA)) in V o is -2- universally V-natural in A for each B 6 B; then an end in V turns out to be simply a family sA: K ~ T(AA) of morphisms in V o which is universally V-natural in A. -
Categorification and (Virtual) Knots
Categorification and (virtual) knots Daniel Tubbenhauer If you really want to understand something - (try to) categorify it! 13.02.2013 = = Daniel Tubbenhauer Categorification and (virtual) knots 13.02.2013 1 / 38 1 Categorification What is categorification? Two examples The ladder of categories 2 What we want to categorify Virtual knots and links The virtual Jones polynomial The virtual sln polynomial 3 The categorification The algebraic perspective The categorical perspective More to do! Daniel Tubbenhauer Categorification and (virtual) knots 13.02.2013 2 / 38 What is categorification? Categorification is a scary word, but it refers to a very simple idea and is a huge business nowadays. If I had to explain the idea in one sentence, then I would choose Some facts can be best explained using a categorical language. Do you need more details? Categorification can be easily explained by two basic examples - the categorification of the natural numbers through the category of finite sets FinSet and the categorification of the Betti numbers through homology groups. Let us take a look on these two examples in more detail. Daniel Tubbenhauer Categorification and (virtual) knots 13.02.2013 3 / 38 Finite Combinatorics and counting Let us consider the category FinSet - objects are finite sets and morphisms are maps between these sets. The set of isomorphism classes of its objects are the natural numbers N with 0. This process is the inverse of categorification, called decategorification- the spirit should always be that decategorification should be simple while categorification could be hard. We note the following observations. Daniel Tubbenhauer Categorification and (virtual) knots 13.02.2013 4 / 38 Finite Combinatorics and counting Much information is lost, i.e.