Homomorphism of a graph

Homomorphism of a graph --- гомоморфизм графа.

For given graphs [math]\displaystyle{ G_{1} }[/math] and [math]\displaystyle{ G_{2} }[/math], a homomorphism from [math]\displaystyle{ G_{1} }[/math] to [math]\displaystyle{ G_{2} }[/math] is a mapping [math]\displaystyle{ \varphi: V(G_{1}) \rightarrow V(G_{2}) }[/math] such that if [math]\displaystyle{ (x,y) \in E(G_{1}) }[/math], then [math]\displaystyle{ (\varphi(x), \varphi(y)) \in E(G_{2}) }[/math].