Let D be a squarefree integer, and let 0 be the ring of integers in the quadratic field Q(√D). For any positive integer f prove that the set Of = Z[fw] = {a + bfw | a, b E Z} is a subring of 0 containing the identity. Prove that [O:Of]= f (index as additive abelian groups). Prove conversely that a subring of 0 containing the identity and having finite index f in 0 (as additive abelian group) is equal to Of. (The ring Of is called the order of conductor f in the field Q(√D). The ring of integers 0 is called the maximal order in Q(√D).