Categories by Horst Schubert (auth.)

By Horst Schubert (auth.)

Categorical tools of talking and pondering have gotten an increasing number of common in arithmetic simply because they in attaining a unifi­ cation of elements of other mathematical fields; often they convey simplifications and supply the impetus for brand new advancements. the aim of this e-book is to introduce the reader to the crucial a part of classification idea and to make the literature available to the reader who needs to move farther. In getting ready the English model, i've got used the chance to revise and amplify the textual content of the unique German version. merely the main simple strategies from set idea and algebra are assumed as necessities. even though, the reader is predicted to be mathe­ to persist with an summary axiomatic process. matically subtle adequate The vastness of the cloth calls for that the presentation be concise, and cautious cooperation and a few endurance is critical at the a part of the reader. Definitions alway precede the examples that remove darkness from them, and it really is assumed that the reader knows a few of the algebraic and topological examples (he are not permit the opposite ones confuse him). it's also was hoping that he'll be capable of clarify the con­ cepts to himself and that he'll realize the motivation.

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Objects and morphisms need not all or always be given specific names, 6. 2 may be read as interrelations between not specifically designated objects and morphisms of a category e. 5 It is clear how diagrams between diagram schemes are defined. This yields a category whosE' objects are diagram schemes and whose morphisms are diagrams. 1 A path w in a diagram scheme L is a finite sequence of arrows aI' a2, ... ,an such that e(ai) = o(ai+l) for i = 1, 2, ... , n - 1. n (> 1) is called the length of w.

If e is a category, then the diagrams of type ElK in t together with their natural transformations form a category, which we denote by [EjK, t]. It is a full subcategory of [E, tJ. 2 Proposition. : be a diagram scheme and let K be a seta! commutativity conditions for I. There exists a (small) category ~(EjK), the path category belonging to E and K, and a diagram L1: I ---+ ~(EjK) with the following universal property: If t is any category, then (i) If D: I ---+ 'e is a diagram 01 type IjK, then there is exactly one functor T D: ~(EjK) ---+ t with D = T D L1.

This functor is represented by the free additive group F with basis M and the inclusion Me U(F). Similar results hold for RMod and Mod R. This example can be regarded as a definition of "free" over M. It can be carried over to other, even non-additive categories, e. g. the category of groups. 10 Theorem. Let e be an additive category. The additive functor e ~ A b is representable if and only if UT: e ~ Ens is representable (U: Ab ~ Ens is the forgetful functor). Proof. Let (A, eA(1 A)) be a representation of UT.

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