Formal Category Theory: Adjointness for 2-Categories

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re Notes in Mathematics Edited by A Dold, Heidelberg and B. Eckmann, Zurich

391 John W. Gray University of Illinois at Urbana-Champaign, Urbana, II/USA

Formal Category Theory: Adjointness for 2-Categories

Springer-Verlag Berlin· Heidelberg· NewYork 1974

AMS Subject Classifications (1970): Primary: 18005, 18025 Secondary: 18A25, 18A40 ISBN 3-540-06830-9 Springer-Verlag Berlin· Heidelberg· New York ISBN 0-387-06830-9 Springer-Verlag New York· Heidelberg· Berlin This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under§ 54 of the German Copyright Law where copies are made for other than private use, a fee is payable to the publisher, the amount of the fee to be determined by agreement with the publisher. © by Springer-Verlag Berlin · Heidelberg 197 4. Library of Congress Catalog Card Number 74·791 0. Printed in Germany. Offsetdruck: Julius Beltz, Hemsbach/Bergstr.

Contents

Introduction !,1

Categories Yoneda

!,2

!,3

11

Adjointness.

12

Fibrations.

12

Adjoint Functor Theorem.

13

Kan ex tens ions.

14

2-Categories.

16

2-functors .

20

Cat-natural transformations

22

Quasi-natural transformations.

25

Modifications ..

28

2-comma category.

29

3-category.

31

3-comma category

32

double category ..

33

2-and 3-categorical fibrations.

35

Bicategories.

38

Pseudo-functors.

40

Quasi-natural transformations.

43

Examples.

45

Biro

45

Spans

46

x.

46

Birr. {B). Biro (Spans Fibrations.

x.l.

48 50

IV I,4

Properties of Fun(A,B} and Pseud (A,B).

55

Quasi-functor of two variables

56

Characterization of Fun(A,B)

59

Composition quasi-functor

67

Quasi-functor of n-variables

69

Tensor product

73

Quasid-natural transformations

80

Quasi -functor.

81

Monoidal closed category structure

83

Pseud

86

X

(A, B)

Appendix li· Universal copseudo-functor.

I,5

Appendix

~.Iso-Fun

Appendix

~.categories

enriched in 2-Cat

92 95 0

Properties of 2-comma categories

101

Universal property.

103

Composition

106

Explicit formulas. Functors over

v1

x

111

v2

Fibration and monoid properties.

I,6

96

115

Homomorphism properties

120 124

Examples

134

Adjoint morphisms in 2-categories

136

Examples

137

Uniqueness, composition and preservation Adjoint Squares

139 144

Examples.

152

Kan extensions

154

Examples

156

Formal criterion for adjoint

158

Cocompleteness.

160

Interchange of limits.

161

Final

163

v I,7

Quasi-adjointness

166

Definitions.

168

Uniqueness, composition and preservation.

169

Transcendental quasi-adjunction.

177

Universal mapping properties.

180

Examples.

187

Some general principles.

187

Some Fini.te quasi-limits.

197

Quasi-colimits in Cat.

201

Quasi-limits in Cat.

217

Quasi-fibrations.

224

Quasi-Kan extensions.

237

The Categorical Comprehension Scheme The Quasi-Yoneda Lemma .

244 251

Globa