Change of Base for Locally Internal Categories
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10. Algebra Theories II 10.1 Essentially Algebraic Theory Idea: A
Page 1 of 7 10. Algebra Theories II 10.1 essentially algebraic theory Idea: A mathematical structure is essentially algebraic if its definition involves partially defined operations satisfying equational laws, where the domain of any given operation is a subset where various other operations happen to be equal. An actual algebraic theory is one where all operations are total functions. The most familiar example may be the (strict) notion of category: a small category consists of a set C0of objects, a set C1 of morphisms, source and target maps s,t:C1→C0 and so on, but composition is only defined for pairs of morphisms where the source of one happens to equal the target of the other. Essentially algebraic theories can be understood through category theory at least when they are finitary, so that all operations have only finitely many arguments. This gives a generalization of Lawvere theories, which describe finitary algebraic theories. As the domains of the operations are given by the solutions to equations, they may be understood using the notion of equalizer. So, just as a Lawvere theory is defined using a category with finite products, a finitary essentially algebraic theory is defined using a category with finite limits — or in other words, finite products and also equalizers (from which all other finite limits, including pullbacks, may be derived). Definition As alluded to above, the most concise and elegant definition is through category theory. The traditional definition is this: Definition. An essentially algebraic theory or finite limits theory is a category that is finitely complete, i.e., has all finite limits. -
Exercise Sheet
Exercises on Higher Categories Paolo Capriotti and Nicolai Kraus Midlands Graduate School, April 2017 Our course should be seen as a very first introduction to the idea of higher categories. This exercise sheet is a guide for the topics that we discuss in our four lectures, and some exercises simply consist of recalling or making precise concepts and constructions discussed in the lectures. The notion of a higher category is not one which has a single clean def- inition. There are numerous different approaches, of which we can only discuss a tiny fraction. In general, a very good reference is the nLab at https://ncatlab.org/nlab/show/HomePage. Apart from that, a very incomplete list for suggested further reading is the following (all available online). For an overview over many approaches: – Eugenia Cheng and Aaron Lauda, Higher-Dimensional Categories: an illustrated guide book available online, cheng.staff.shef.ac.uk/guidebook Introductions to simplicial sets: – Greg Friedman, An Elementary Illustrated Introduction to Simplicial Sets very gentle introduction, arXiv:0809.4221. – Emily Riehl, A Leisurely Introduction to Simplicial Sets a slightly more advanced introduction, www.math.jhu.edu/~eriehl/ssets.pdf Quasicategories: – Jacob Lurie, Higher Topos Theory available as paperback (ISBN-10: 0691140499) and online, arXiv:math/0608040 Operads (not discussed in our lectures): – Tom Leinster, Higher Operads, Higher Categories available as paperback (ISBN-10: 0521532159) and online, 1 arXiv:math/0305049 Note: Exercises are ordered by topic and can be done in any order, unless an exercise explicitly refers to a previous one. We recommend to do the exercises marked with an arrow ⇒ first. -
HIGHER-DIMENSIONAL ALGEBRA V: 2-GROUPS 1. Introduction
Theory and Applications of Categories, Vol. 12, No. 14, 2004, pp. 423–491. HIGHER-DIMENSIONAL ALGEBRA V: 2-GROUPS JOHN C. BAEZ AND AARON D. LAUDA Abstract. A 2-group is a ‘categorified’ version of a group, in which the underlying set G has been replaced by a category and the multiplication map m: G × G → G has been replaced by a functor. Various versions of this notion have already been explored; our goal here is to provide a detailed introduction to two, which we call ‘weak’ and ‘coherent’ 2-groups. A weak 2-group is a weak monoidal category in which every morphism has an ∼ ∼ inverse and every object x has a ‘weak inverse’: an object y such that x ⊗ y = 1 = y ⊗ x. A coherent 2-group is a weak 2-group in which every object x is equipped with a specified weak inversex ¯ and isomorphisms ix:1→ x⊗x¯, ex:¯x⊗x → 1 forming an adjunction. We describe 2-categories of weak and coherent 2-groups and an ‘improvement’ 2-functor that turns weak 2-groups into coherent ones, and prove that this 2-functor is a 2-equivalence of 2-categories. We internalize the concept of coherent 2-group, which gives a quick way to define Lie 2-groups. We give a tour of examples, including the ‘fundamental 2-group’ of a space and various Lie 2-groups. We also explain how coherent 2-groups can be classified in terms of 3rd cohomology classes in group cohomology. Finally, using this classification, we construct for any connected and simply-connected compact simple Lie group G a family of 2-groups G ( ∈ Z)havingG as its group of objects and U(1) as the group of automorphisms of its identity object. -
Higher Dimensional Algebra VI: Lie 2-Algebra
Mathematics Faculty Works Mathematics 2004 Higher Dimensional Algebra VI: Lie 2-Algebra John C. Baez University of California, Riverside Alissa S. Crans Loyola Marymount University, [email protected] Follow this and additional works at: https://digitalcommons.lmu.edu/math_fac Part of the Algebra Commons Recommended Citation Baez, J. and Crans, A. “Higher Dimensional Algebra VI: Lie 2-Algebras.” Theory and Applications of Categories, Vol. 12 (2004): 492 – 538. This Article is brought to you for free and open access by the Mathematics at Digital Commons @ Loyola Marymount University and Loyola Law School. It has been accepted for inclusion in Mathematics Faculty Works by an authorized administrator of Digital Commons@Loyola Marymount University and Loyola Law School. For more information, please contact [email protected]. Theory and Applications of Categories, Vol. 12, No. 15, 2004, pp. 492{538. HIGHER-DIMENSIONAL ALGEBRA VI: LIE 2-ALGEBRAS JOHN C. BAEZ AND ALISSA S. CRANS Abstract. The theory of Lie algebras can be categorified starting from a new notion of `2-vector space', which we define as an internal category in Vect. There is a 2- category 2Vect having these 2-vector spaces as objects, `linear functors' as morphisms and `linear natural transformations' as 2-morphisms. We define a `semistrict Lie 2- algebra' to be a 2-vector space L equipped with a skew-symmetric bilinear functor [·; ·]: L×L ! L satisfying the Jacobi identity up to a completely antisymmetric trilinear natural transformation called the `Jacobiator', which in turn must satisfy a certain law of its own. This law is closely related to the Zamolodchikov tetrahedron equation, and indeed we prove that any semistrict Lie 2-algebra gives a solution of this equation, just as any Lie algebra gives a solution of the Yang{Baxter equation. -
Apr 0 8 2011
From manifolds to invariants of En-algebras MASSACHUSETTS INSTITUTE by OF TECHNIOLOGY Ricardo Andrade APR 08 2011 Licenciatura, Instituto Superior Tecnico (2005) LjBRARIES Submitted to the Department of Mathematics in partial fulfillment of the requirements for the degree of Doctor of Philosophy ARCHIVEs at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY September 2010 @ Ricardo Andrade, MMX. This work is licensed under the Creative Commons Attribution-ShareAlike 3.0 License. To view a copy of this license, visit http: //creativecommons .org/licenses/by-sa/3.0/legalcode The author hereby grants to MIT permission to reproduce and distribute publicly paper and electronic copies of this thesis document in whole or in part. Author .......... Department of Mathematics August 24, 2010 Certified by.... Haynes R. Miller Professor of Mathematics Thesis Supervisor 7) Accepted by ... 6$, Bjorn Poonen Chairman, Department Committee on Graduate Students From manifolds to invariants of En-algebras by Ricardo Andrade Submitted to the Department of Mathematics on August 24, 2010, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Abstract This thesis is the first step in an investigation of an interesting class of invariants of En-algebras which generalize topological Hochschild homology. The main goal of this thesis is to simply give a definition of those invariants. We define PROPs EG, for G a structure group sitting over GL(n, R). Given a manifold with a (tangential) G-structure, we define functors 0 EG[M]: (EG) -+ Top constructed out of spaces of G-augmented embeddings of disjoint unions of euclidean spaces into M. These spaces are modifications to the usual spaces of embeddings of manifolds. -
INTERNAL CATEGORIES, ANAFUNCTORS and LOCALISATIONS in Memory of Luanne Palmer (1965-2011)
Theory and Applications of Categories, Vol. 26, No. 29, 2012, pp. 788{829. INTERNAL CATEGORIES, ANAFUNCTORS AND LOCALISATIONS In memory of Luanne Palmer (1965-2011) DAVID MICHAEL ROBERTS Abstract. In this article we review the theory of anafunctors introduced by Makkai and Bartels, and show that given a subcanonical site S, one can form a bicategorical localisation of various 2-categories of internal categories or groupoids at weak equivalences using anafunctors as 1-arrows. This unifies a number of proofs throughout the literature, using the fewest assumptions possible on S. 1. Introduction It is a well-known classical result of category theory that a functor is an equivalence (that is, in the 2-category of categories) if and only if it is fully faithful and essentially surjective. This fact is equivalent to the axiom of choice. It is therefore not true if one is working with categories internal to a category S which doesn't satisfy the (external) axiom of choice. This is may fail even in a category very much like the category of sets, such as a well-pointed boolean topos, or even the category of sets in constructive foundations. As internal categories are the objects of a 2-category Cat(S) we can talk about internal equivalences, and even fully faithful functors. In the case S has a singleton pretopology J (i.e. covering families consist of single maps) we can define an analogue of essentially surjective functors. Internal functors which are fully faithful and essentially surjective are called weak equivalences in the literature, going back to [Bunge-Par´e1979]. -
Complete Internal Categories
Complete Internal Categories Enrico Ghiorzi∗ April 2020 Internal categories feature notions of limit and completeness, as originally proposed in the context of the effective topos. This paper sets out the theory of internal completeness in a general context, spelling out the details of the definitions of limit and completeness and clarifying some subtleties. Remarkably, complete internal categories are also cocomplete and feature a suitable version of the adjoint functor theorem. Such results are understood as consequences of the intrinsic smallness of internal categories. Contents Introduction 2 1 Background 3 1.1 Internal Categories . .3 1.2 Exponentials of Internal Categories . .9 2 Completeness of Internal Categories 12 2.1 Diagrams and Cones . 12 2.2 Limits . 14 2.3 Completeness . 17 2.4 Special Limits . 20 2.5 Completeness and Cocompleteness . 22 arXiv:2004.08741v1 [math.CT] 19 Apr 2020 2.6 Adjoint Functor Theorem . 25 References 32 ∗Research funded by Cambridge Trust and EPSRC. 1 Introduction Introduction It is a remarkable feature of the effective topos [Hyl82] to contain a small subcategory, the category of modest sets, which is in some sense complete. That such a category should exist was originally suggested by Eugenio Moggi as a way to understand how realizability toposes give rise to models for impredicative polymorphism, and concrete versions of such models already appeared in [Gir72]. The sense in which the category of modest sets is complete is a delicate matter which, among other aspects of the situation, is considered in [HRR90]. This notion of (strong) completeness is related in a reasonably straightforward way—using the externalization of an internal category—to the established notion for indexed categories [PS78]. -
Higher Categories Student Seminar
Higher categories student seminar Peter Teichner and Chris Schommer-Pries Contents 0 Introduction { Peter Teichner and Chris Schommer-Pries 2 0.1 Part 1: Lower higher categories { Peter . .2 0.1.1 Motivation 1: Higher categories show up everywhere . .2 0.1.2 Motivation 2: The homotopy hypothesis . .4 0.2 Part 2: Higher higher categories { Chris . .5 0.2.1 Broad outline and motivation . .5 0.2.2 Detailed outline . .6 1 Strict n-categories and their classifying spaces { Lars Borutzky 6 1.1 Strict n-categories, left Kan extensions, and nerves of 1-categories . .6 1.2 Higher nerve functors . .8 1.3 Examples of higher categories . .9 2 Strictification of weak 2-categories { Arik Wilbert 10 2.1 Motivation . 10 2.2 The theorem . 11 2.2.1 The definitions . 11 2.2.2 The proof . 13 2.3 Correction from previous lecture . 14 3 2-groupoids and 2-types { Malte Pieper 15 3.1 Motivation . 15 3.2 What's a 2-group? . 15 3.3 Classify 2-groups! . 16 3.4 2-types/equivalence $ 2-groups/equivalence . 17 3.5 Addendum (handout): Classification of homotopy 2-types: Some basic definitions . 19 4 No strict 3-groupoid models the 2-sphere { Daniel Br¨ugmann 21 4.1 Outline . 21 4.2 The homotopy groups of groupoids . 22 4.3 The auxiliary result . 23 5 Complete Segal spaces and Segal categories { Alexander K¨orschgen 24 1 5.1 Definitions . 24 5.2 Groupoids . 26 5.3 Relation with n-Types ........................................ 26 5.4 Examples . 27 6 Examples and problems with weak n-categories { Kim Nguyen 27 6.1 Background . -
Higher-Dimensional Algebra Vi: Lie 2-Algebras
Theory and Applications of Categories, Vol. 12, No. 15, 2004, pp. 492{538. HIGHER-DIMENSIONAL ALGEBRA VI: LIE 2-ALGEBRAS JOHN C. BAEZ AND ALISSA S. CRANS Abstract. The theory of Lie algebras can be categorified starting from a new notion of `2-vector space', which we define as an internal category in Vect. There is a 2- category 2Vect having these 2-vector spaces as objects, `linear functors' as morphisms and `linear natural transformations' as 2-morphisms. We define a `semistrict Lie 2- algebra' to be a 2-vector space L equipped with a skew-symmetric bilinear functor [·; ·]: L×L ! L satisfying the Jacobi identity up to a completely antisymmetric trilinear natural transformation called the `Jacobiator', which in turn must satisfy a certain law of its own. This law is closely related to the Zamolodchikov tetrahedron equation, and indeed we prove that any semistrict Lie 2-algebra gives a solution of this equation, just as any Lie algebra gives a solution of the Yang{Baxter equation. We construct a 2- category of semistrict Lie 2-algebras and prove that it is 2-equivalent to the 2-category of 2-term L1-algebras in the sense of Stasheff. We also study strict and skeletal Lie 2-algebras, obtaining the former from strict Lie 2-groups and using the latter to classify Lie 2-algebras in terms of 3rd cohomology classes in Lie algebra cohomology. This classification allows us to construct for any finite-dimensional Lie algebra g a canonical 1-parameter family of Lie 2-algebras g~ which reduces to g at ~ = 0. -
Cosmoi of Internal Categories by Ross Street
TRANSACTIONS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 258, Number 2, April 1980 COSMOI OF INTERNAL CATEGORIES BY ROSS STREET Abstract. An internal full subcategory of a cartesian closed category & is shown to give rise to a structure on the 2-category Cat(&) of categories in & which introduces the notion of size into the analysis of categories in & and allows proofs by transcendental arguments. The relationship to the currently popular study of locally internal categories is examined. Internal full subcategories of locally presentable categories (in the sense of Gabriel-Ulmer) are studied in detail. An algorithm is developed for their construc- tion and this is applied to the categories of double categories, triple categories, and so on. Introduction. The theory of categories is essentially algebraic in the terminology of Freyd [12]. This means that it is pertinent to take models of the theory in any finitely complete category &. Of course this does not mean that all the usual properties of categories are available to us in &. The 2-category Cat{&) of categories in & is finitely complete (Street [29]) so pullbacks, categories of Eilen- berg-Moore algebras for monads, comma categories, and so on, are available. Even some colimit constructions such as the categories of Kleisli algebras exist. Obvi- ously the more like the category Set of sets â becomes the more closely category theory internal to 6E resembles ordinary category theory. If & is finitely cocomp- lete, for example, we gain localization; or, if 6Bis cartesian closed we gain functor categories. Yet there is an aspect of category theory which distinguishes it from other essentially algebraic theories; namely, the question of size. -
An Informal Introduction to Topos Theory
An informal introduction to topos theory Tom Leinster∗ Contents 1 The definition of topos 3 2 Toposes and set theory 7 3 Toposes and geometry 13 4 Toposes and universal algebra 22 This short text is for readers who are confident in basic category theory but know little or nothing about toposes. It is based on some impromptu talks given to a small group of category theorists. I am no expert on topos theory. These notes are for people even less expert than me. In keeping with the spirit of the talks, what follows is light on both detail and references. For the reader wishing for more, almost everything here is presented in respectable form in Mac Lane and Moerdijk’s very pleasant introduction to topos theory (1994). Nothing here is new, not even the expository viewpoint (very loosely inspired by Johnstone (2003)). As a rough indication of the level of knowledge assumed, I will take it that arXiv:1012.5647v3 [math.CT] 27 Jun 2011 you are totally comfortable with the Yoneda Lemma and the concept of cartesian closed category, but I will not assume that you know the definition of subobject classifier or of topos. Section 1 explains the definition of topos. The remaining three sections discuss some of the connections between topos theory and other subjects. There are many more such connections than I will mention; I hope it is abundantly clear that these notes are, by design, a quick sketch of a large subject. Section 2 is on connections between topos theory and set theory. There are two themes here. -
Categories in Categories, and Size Matters
Categories in categories, and size matters Ross Street∗ 2010 Mathematics Subject Classification 18D10; 18D20; 18D35 Key words and phrases. internal category; category object; internal full subcategory; fibration; double category; descent category; distributors; profunctors; modules; Yoneda structure. Abstract We look again at internal categories and internal full subcategories in a category C . We look at the relationship between a generalised Yoneda lemma and the descent construction. Application to C = Cat gives results on double categories. Introduction We revisit the theory of categories in a category. We look in particular at categories in the category of categories; that is, at double categories. Contents 1 Categories in categories 1 2 Fibrations and the generalized Yoneda Lemma 4 3 Internal full subcategories 9 4 Slices and left extensions 14 ∗The author gratefully acknowledges the support of Australian Research Council Discovery Grants DP1094883 and DP130101969. 1 5 Descent categories 18 6 Descent and codescent objects 21 7 Split fibrations in a category 26 8 Two-sided discrete fibrations 30 9 Size and cocompleteness 33 10 Internal full subcategories of Cat 41 1 Categories in categories A category C in a category C is a diagram d0 / d0 / o i0 d0 / d1 / C C3 C2 d1 / C1 o i C0 (1.1) ∶ d2 / o i1 d1 / d3 / d2 / in C such that C1. dpdq = dq−1dp for p < q C2. d0i = d1i = 1C0 ; d0i0 = d1i0 = d1i1 = d2i1 = 1C1 C3. i0i = i1i; d2i0 = id1; d0i1 = id0 C4. the following two squares are pullbacks. d0 d0 C2 / C1 C3 / C2 d2 d1 d3 d2 (1.2) C1 / C0 C2 / C1 d0 d0 The definition goes back to Ehresmann [11].