Therefore,
both x1 and x2 must be either odd or even Suppose both x1 and x2 are odd Then
f (x1) = f (x2) ⇒ x1 + 1 = x2 + 1 ⇒ x1 = x2 Similarly, if both x1 and x2 are even, then also
f (x1) = f (x2) ⇒ x1 – 1 = x2 – 1 ⇒ x1 = x2