# Basic Category Theory by Tom Leinster

By Tom Leinster

On the middle of this brief creation to class idea is the belief of a common estate, very important all through arithmetic. After an introductory bankruptcy giving the fundamental definitions, separate chapters clarify 3 ways of expressing common homes: through adjoint functors, representable functors, and bounds. a last bankruptcy ties all 3 jointly. The ebook is acceptable to be used in classes or for self sustaining learn. Assuming quite little mathematical heritage, it's perfect for starting graduate scholars or complex undergraduates studying class conception for the 1st time. for every new specific proposal, a beneficiant offer of examples is equipped, taken from diversified elements of arithmetic. At issues the place the bounce in abstraction is especially nice (such because the Yoneda lemma), the reader will locate cautious and vast motives. Copious workouts are integrated.

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**Example text**

But in this book, we climb no higher than the first rung or two of this infinite ladder. 25 Find three examples of natural transformations not mentioned above. 11. 27 Let A and B be categories. Prove that [A op , B op ] 39 [A , B]op . 28 Let A and B be sets, and denote by BA the set of functions from A to B. Write down: (a) a canonical function A × BA → B; A (b) a canonical function A → B(B ) . 29 Here we consider natural transformations between functors whose domain is a product category A × B.

27). 1, F is faithful if for each A, A and g as shown, there is at most one dotted arrow that F sends to g. It is full if for each such A, A and g, there is at least one dotted arrow that F sends to g. 18 Let A be a category. A subcategory S of A consists of a subclass ob(S ) of ob(A ) together with, for each S , S ∈ ob(S ), a subclass S (S , S ) of A (S , S ), such that S is closed under composition and identities. It is a full subcategory if S (S , S ) = A (S , S ) for all S , S ∈ ob(S ). A full subcategory therefore consists of a selection of the objects, with all of the maps between them.

Such operations are called contravariant functors. 10 Let A and B be categories. A contravariant functor from A to B is a functor A op → B. To avoid confusion, we write ‘a contravariant functor from A to B’ rather than ‘a contravariant functor A → B’. Functors C → D correspond one-to-one with functors C op → D op , and (A op )op = A , so a contravariant functor from A to B can also be described as a functor A → B op . 10 makes life easier. An ordinary functor A → B is sometimes called a covariant functor from A to B, for emphasis.