I need to give examples of functions mapping from the natural numbers into the natural numbers with the following properties:

1. Surjective, not injective

2. Injective, not surjective

f(x) = 2x

3. Neither injective nor surjective

4. Bijective

f(x) = x

I think I figured out (2) and (4) but any help with the other two conditions would be helpful!