Definition
Associativity allows us to write an expression of the form x1 · x2 · · · xn (where
x1, · · · , xn ∈ G) without ambiguity. Let x ∈ G. For any positive integer n, define
$$x^n = x \cdot x \cdot \cdot \cdot x.$$
n times
Then define x−n = (x−1)n and x0 = e.
Exercise
Let x ∈ G, and let m and n be integers. Prove the following.
1. x−n = (xn)−1
2. xm · xn = xm+n
3. (xm)n = xmn
Note: In each case, m and n may (independently) be positive, negative, or zero. Since the definition of xn is
different when n is positive, negative, and zero, the proof must deal with all these cases separately.