Def/Ring homomorphism



=Preservation= In addition to its defining properties, a ring homomorphism $f \colon R \rightarrow S$ satisfies the following:
 * For all $r_1, r_2 \in R$, $f(r_1 - r_2) = f(r_1) - f(r_2)$.
 * $f(0) = 0$.

=Metadata=