3. The function g:ZZZ given by g(mn)=2m+3n is surjective but it is not injective. To show surjectivity, note that if k=2l then k=g(l,0); and if k= 2l+1 then k=2(l1)+3=g(l1,1). To show that g is not injective, for example, observe that g(3,2)= g(6,0)=g(0,4)