Consider the ring homomorphism ϕ : Z[x] →R defined by ϕ(x) =
√5.
Let I = {f ∈Z[x]|ϕ(f) = 0}.
First prove that I is an ideal in Z[x]. Then find g ∈ Z[x] such
that I = (g). [You do not need to prove the last equality.]
Prove the following:
(a) Let A be a ring and B be a field. Let f : A → B be a
surjective homomorphism from A to B. Then ker(f) is a maximal
ideal.
(b) If A/J is a field, then J is a maximal ideal.