Restriction of endomorphism to invariant subgroup is endomorphism
Statement
Suppose is a group, is a subgroup and is an endomorphism of with the property that for all . Then, the restriction of to defines an endomorphism of .
Suppose  is a group, 
 is a subgroup and 
 is an endomorphism of 
 with the property that 
 for all 
. Then, the restriction of 
 to 
 defines an endomorphism of 
.