Ring Homomorphism Preserves Zero

Theorem
Let $\phi: \struct {R_1, +_1, \circ_1} \to \struct {R_2, +_2, \circ_2}$ be a ring homomorphism.

Let:
 * $0_{R_1}$ be the zero of $R_1$
 * $0_{R_2}$ be the zero of $R_2$.

Then:
 * $\map \phi {0_{R_1} } = 0_{R_2}$

Proof
By definition, if $\struct {R_1, +_1, \circ_1}$ and $\struct {R_2, +_2, \circ_2}$ are rings then $\struct {R_1, +_1}$ and $\struct {R_2, +_2}$ are groups.

Again by definition:
 * the zero of $\struct {R_1, +_1, \circ_1}$ is the identity of $\struct {R_1, +_1}$
 * the zero of $\struct {R_2, +_2, \circ_2}$ is the identity of $\struct {R_2, +_2}$.

The result follows from Group Homomorphism Preserves Identity.