By Derek J. S. Robinson

This undergraduate textbook for a two-semester direction in summary algebra lightly introduces the primary constructions of contemporary algebra. Robinson (University of Illinois) defines the suggestions at the back of units, teams, subgroups, teams performing on units, earrings, vector areas, box concept, and Galois concept

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4), and it is denoted by X. Obviously X is the smallest subgroup of G containing X: it is called the subgroup generated by X. Note that the cyclic subgroup x is just the subgroup generated by the singleton set {x}. Thus we have generalized the notion of a cyclic subgroup. It is natural to enquire what form the elements of X take. 5) Let X be a non-empty subset of a group G. Then X consists of all elements of G of the form x1ε1 x2ε2 . . xkεk where xi ∈ X, εi = ±1 and k ≥ 0, (the case k = 0 being interpreted as 1G ).

Prove that (P (A), ∗) is an abelian group. 5. Define powers in a semigroup (S, ∗) by the rules x 1 = x and x n+1 = x n ∗ x where x ∈ S and n is a non-negative integer. Prove that x m ∗ x n = x m+n and (x m )n = x mn where m, n > 0. 6. Let G be a monoid such that for each x in G there is a positive integer n such that x n = e. Prove that G is a group. 7. Let G consist of the permutations (1 2)(3 4), (1 3)(2 4), (1 4)(2 3), together with the identity permutation (1)(2)(3)(4). Show that G is a group with exactly four elements in which each element is its own inverse.

Vi) For an example of a semigroup that is not a monoid we need look no further than (E, +) where E is the set of all even integers. Clearly there is no identity element here. (vii) The monoid of functions on a set. Let A be any non-empty set, and write Fun(A) for the set of all functions α on A. 3). The identity element is the identity function on A. , to those which have inverses, we obtain the symmetric group on A (Sym(A), ), consisting of all the permutations of A. This is one of our prime examples of groups.