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 Thus, f is one-one Also, any odd number
2r + 1 in the co-domain N is the image of 2r + 2 in the domain N and any even number
2r in the co-domain N is the image of 2r – 1 in the domain N