When is a subgroup of a ring an ideal?

Sunil K. Chebolu, Christina L. Henry

Let $R$ be a commutative ring. When is a subgroup of $(R,+)$ an ideal of $R$? We investigate this problem for the rings ${\mathbb{Z}}^d$ and ${\prod }_{i=1}^d{\mathbb{Z}}_{n_i}$.…