Let R = real numbers, Z = integers, N = natural numbers, Q = rational numbers then which of the following is not a semigroup?
I. (N, +)
II. (Z, +)
III. (Z, -)
IV. (Q, -)
1
I and II
2
II and III
3
III and IV
4
IV and I
Let R = real numbers, Z = integers, N = natural numbers, Q = rational numbers then which of the following is not a semigroup?
I. (N, +)
II. (Z, +)
III. (Z, -)
IV. (Q, -)