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a) the set of odd positive integers

b) the set of positive integer powers of 3

c) the set of polynomials with integer coefficients

I have the first two:

a) f(0)=1, f(n)=f(n-1)+2 for n>=1

b) f(0)=1, f(n)=3f(n-1) for n>=1

For c, I am not even quite sure exactly what it is asking or where to begin?