(PRIME IDEALS IN Z[X]) Consider the ring Z[X].
(a) Consider the map ϕ : Z[X] → Z given by f ↦ f(0), which is obviously a ring homomorphism. Show that ker ϕ = (X).
(b) Use part (a) and the First Isomorphism Theorem to show that (X) is a prime ideal of Z[X] that is not maximal.
(c) Show that (2, X) = {f ∈ Z[X] : f(0) is even}.
(d) Consider the map ϕ : Z[X] → Z₂ given by f ↦ f(0) mod 2 (where Z₂ = Z/2Z is the field of two elements), which is also a ring homomorphism. Show that ker ϕ = (2, X).
(e) Show that (2, X) is a maximal ideal of Z[X].
(f) Show that (2, X) is not a principal ideal of Z[X]. (Hint: Suppose to obtain a contradiction that (2, X) = (f) for some f ∈ Z[X]. Use part (c) to try and derive a contradiction.)