In the following determine whether the systems described are groups. If they are not, point out which of the group axioms fail to hold.
(a) G = set of all integers, a· b = a - b.
(b) G = set of all positive integers, a · b = ab, the usual product of integers.
(c) G = a0 , a 1 , ... , a6 where ai · a i = ai + i if i + j < 7, ai · a i = ai + i _ 7 if i + j ;:::: 7 (for instance, a5 • a 4 = a 5 +4 _ 7 = a2 since 5 + 4 = 9 > 7).
(d) G = set of all rational numbers with odd denominators, a· b _ a + b, the usual addition of rational numbers.
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