Let
D and D'be integral domains.
Let c = charD and c'= charD'
(a) Prove that the direct product D ×D'has unity.
(b) Let a ∈D and b∈D'.
Prove that (a, b) is a unit in D ×D'⇐⇒
a is a unit in D, b is a unit in D'.
(c) Prove that D×D'is never an integral domain.
(d) Prove that if c, c'> 0,
then char(D ×D') = lcm(c, c')
(e) Prove that if c = 0, then char(D...