Basic Category Theory by Tom Leinster

By Tom Leinster

On the middle of this brief advent to class conception is the assumption of a common estate, very important all through arithmetic. After an introductory bankruptcy giving the elemental 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 booklet is acceptable to be used in classes or for self reliant research. Assuming quite little mathematical history, it's perfect for starting graduate scholars or complex undergraduates studying classification concept for the 1st time. for every new express suggestion, a beneficiant provide of examples is supplied, taken from varied elements of arithmetic. At issues the place the jump in abstraction is especially nice (such because the Yoneda lemma), the reader will locate cautious and huge motives. Copious routines are integrated.

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Basic Category Theory

On the middle of this brief advent to type idea is the belief of a common estate, very important all through arithmetic. After an introductory bankruptcy giving the elemental definitions, separate chapters clarify 3 ways of expressing common homes: through adjoint functors, representable functors, and boundaries.

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Similar constructions will be crucial in later chapters. For certain classes of space, the passage from X to C(X) loses no information: there is a way of reconstructing the space X from the ring C(X). For this and related reasons, it is sometimes said that ‘algebra is dual to geometry’. 12 Let k be a field. For any two vector spaces V and W over k, there is a vector space Hom(V, W) = {linear maps V → W}. The elements of this vector space are themselves maps, and the vector space operations (addition and scalar multiplication) are defined pointwise, as in the last example.

Precisely, it is a function α : S → T such that α(g · s) = g · α(s) for all s ∈ S and g ∈ G. ) In other words, it is just a map of G-sets, sometimes called a G-equivariant map. 5 Fix a natural number n. In this example, we will see how ‘determinant of an n × n matrix’ can be understood as a natural transformation. For any commutative ring R, the n × n matrices with entries in R form a monoid Mn (R) under multiplication. Moreover, any ring homomorphism R → S induces a monoid homomorphism Mn (R) → Mn (S ).

Thus, the value of Hom(−, W) at V is Hom(V, W). Sometimes we use a blank space instead of −, as in Hom( , W). An important special case is where W is k, seen as a one-dimensional vector space over itself. The vector space Hom(V, k) is called the dual of V, and is written as V ∗ . So there is a contravariant functor ( )∗ = Hom(−, k) : Vectk → Vectk op sending each vector space to its dual. 13 For each n ∈ N, there is a functor H n : Topop → Ab assigning to a space its nth cohomology group. 14 Let G be a monoid, regarded as a one-object category G .

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