Subdomain Test

Theorem
Let $S$ be a subset of an integral domain $\struct {R, +, \circ}$.

Then $\struct {S, +\restriction_S, \circ \restriction_S}$ is a subdomain of $\struct {R, +, \circ}$ these conditions hold:


 * $(1): \quad$ $\struct {S, + \restriction_S, \circ \restriction_S}$ is a subring of $\struct {R, +, \circ}$
 * $(2): \quad$ The unity of $R$ is also in $S$, that is $1_R = 1_S$.

Proof
By Idempotent Elements of Ring with No Proper Zero Divisors, it follows that the unity of a subdomain is the unity of the integral domain it's a subdomain of.