CHAPTER I.
Introduction.
§0. General prerequisites.
In this work we shall assume familiarity with the elementary concepts of
noncommutative ring theory, including that of a tensor product of bimodules, and
with the basic terminology of category theory, including ''natural transformations"
(morphisms of functors). General references for ring theory are [115], [48], [49],
for category theory, [122].
More specialized results and concepts which are important in this work, in
particular, those of algebra and coalgebra objects in a category, and of representable
algebra-valued functors, are developed in Chapter II. Below, in §1, we give a brief
motivated sketch of some of this background material, followed in §2 by a summary
of the main results of Chapters III, V, and VI, and some indication of the subject
matter of the remaining chapters.
§1. Introductory sketch - what are coalgebras, and why?
Clearly, the following is a very basic type of construction in algebra: Given two
categories of algebraic objects C and D (e.g., commutative rings, and groups),
we construct from each object S of C an object V(S) of D by taking for the
elements of V(S) all X-tuples of elements of S (X a fixed set) that satisfy a fixed
system Y of equations in their coordinates, and defining the operations of V(S)
by formulas (in precise language, "terms") in these coordinates under the
operations of S. Examples include "forgetful functors", where X = l , Y= 0
and the operations of V(S) are a subset (possibly empty) of the operations of 5,
as well as less trivial constructions, such as the formal-power-series-ring functor
from rings to rings, and the functor SLn from commutative rings to groups. Note
that to establish that one has such a construction, one must verify that the operations
so defined will always take a family of Z-tuples satisfying the equations Y to an
X-tuple again satisfying these equations, and that the resulting objects V(S) will
satisfy the identities or other conditions defining membership in D.
A key tool in studying such functors is the following observation. Let
categories C and D, and a construction V based, as above, on families X and
Y be given. Then if C is a reasonable category of algebraic objects (e.g., a
variety of algebras, definition recalled in §6 below), we can form an object of C
presented by X and Y as generators and relations:
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http://dx.doi.org/10.1090/surv/045/01
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