Definition:Homomorphism (Abstract Algebra)/Cartesian Product

Definition
Let $\phi: \struct {S, \circ} \to \struct {T, *}$ be a mapping from one algebraic structure $\struct {S, \circ}$ to another $\struct {T, *}$.

We define the cartesian product $\phi \times \phi: S \times S \to T \times T$ as:
 * $\forall \tuple {x, y} \in S \times S: \map {\paren {\phi \times \phi} } {x, y} = \tuple {\map \phi x, \map \phi y}$

Hence we can state that $\phi$ is a homomorphism :
 * $\map \ast {\map {\paren {\phi \times \phi} } {x, y} } = \map \phi {\map \circ {x, y} }$

using the notation $\map \circ {x, y}$ to denote the operation $x \circ y$.

The point of doing this is so we can illustrate what is going on in a commutative diagram:


 * $\begin{xy} \xymatrix@L+2mu@+1em{

S \times S \ar[r]^*{\circ} \ar[d]_*{\phi \times \phi} & S \ar[d]^*{\phi} \\ T \times T \ar[r]_*{\ast} & T }\end{xy}$

Thus we see that $\phi$ is a homomorphism iff both of the composite mappings from $S \times S$ to $T$ have the same effect on all elements of $S \times S$.