2. (a) Show that every ideal in ring Z is principal. More specifi-cally, prove the following: if A is an ideal in Z, then A = (n) = nZ, where n is the smallest positive integer in A, or A = {0}, if A contains no positive integers. [Remark: Integral domains in which every ideal is principal are called principal ideal domains. In addition to Z, an example of a principal ideal domain is R[x].] (b) Suppose that F is a field and there is a ring homomorphism from Z onto F. Show that F is isomorphic to Zp, for some prime p.
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If \( A = \{0\} \), then \( A = (0) = 0\mathbb{Z} \) and we are done. ** Show more…
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(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.)
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