Let A and B be commutative rings with unity where char(A)=n and Char(B)=m s.t. n,m ∈ ℤ (and n,m ≠ 0). Prove or give counter example: if k ∈ ℤ+ and n,m both divide k, then Char (A x B)| k.
Here was my thinking but I don't know if it is true: Char(A x B) = gcd(n,m) . I know that if n|k and m|k, we want to show that gcd(n,m)|K. However, I don't know if my initial assumption that Char(A x B) = gcd(n,m) is actually correct.
I know that a characteristic of a ring with unity is the smallest such n st 1*n≡ 0 where 1 is the multiplicative identity. I tried to use an easy example:
Char(ℤ12) = 12, Char(ℤ6)=6, Char(ℤ12 x ℤ6) = 3 or does it equal 6???