Ok the question is:

Give an example of a function from N to N that is

(a) one-to-one but not onto

(b) onto but not one-to-one

(c) both onto and one-to-one

(d) neither one-to-one nor onto

(a) My answer is the function from {a,b,c} to {1,2,3,4} with f(a) = 2, f(b) = 3, f(c) = 1. Is this the correct example to this question?

What does it mean from N to N?

I kind of lost on how to show these examples. Thanks for any help!