if m is odd and n is odd, then mn is odd.

if m is odd and n is odd, then mn is odd.

m = 2k +a where a = 0 or 1

n = 2l + b where b = 0 or 1

mn = (2k + a)(2l + b) = 4kl + 2kb + 2al + ab

Since mn is odd, ab = 1 since a = 1 and b = 1