Exercise 1.2.8. Here are two important definitions related to a function f : A - B. The function f is one-to-one (1-1) if a1 * a2 in A implies that f(al) # f(a2) in B. The function f is onto if, given any b E B, it is possible to find an element a E A for which f(a) = b. Give an example of each or state that the request is impossible: (a) f : N - N that is 1-1 but not onto. (b) f : N - N that is onto but not 1-1. (c) f : N - Z that is 1-1 and onto