Let (I,+, ·) be an integral domain. Consider the set {(a,b) : a,b ∈ I and b ≠ 0_I}. There is an equivalence relation on this set given by (a,b) ≅ (c,d) if ad = bc. Let [(a,b)] denote the equivalency class of (a,b). Define R := {[(a,b)] : a,b ∈ I and b ≠ 0_I} and
• [(a,b)] + [(c,d)] = [(ad + bc, bd)];
• [(a,b)] · [(c,d)] = [(ac, bd)].
It is known that (R, +, ·) is a ring. I want you to check some of the ring axioms below in addition to other properties that R might have. Hint: Think about this in terms of I = ℤ and R = ℚ for an example.
(a) Show that addition + is associative.
(b) What is 0_R?
(c) What is 1_R?
(d) Show that multiplication · is commutative.
(e) Show that the nonzero elements of R are units
(f) Conclude that every integral domain is a subring of some field.