Fully invariant implies characteristic

This article gives the statement and possibly, proof, of an implication relation between two subgroup properties. That is, it states that every subgroup satisfying the first subgroup property (i.e., fully invariant subgroup) must also satisfy the second subgroup property (i.e., characteristic subgroup)
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Statement

Any fully invariant subgroup (also called fully characteristic subgroup) of a group is a characteristic subgroup.

Definitions used

Fully invariant subgroup

Further information: fully invariant subgroup

A subgroup $H$ of a group $G$ is termed fully invariant in $G$ if, for every endomorphism $\phi$ of $G$, $\phi (H)$ is contained in $H$.

Characteristic subgroup

Further information: characteristic subgroup

A subgroup $H$ of a group $G$ is termed characteristic in $G$ if, for every automorphism $\phi$ of $G$, $\phi (H)$ is contained in $H$.

Proof

Proof idea

The idea behind the proof is that since every automorphism is an endomorphism, invariance under all endomorphisms implies invariance under automorphisms.

Using function restriction expressions

This subgroup property implication can be proved by using function restriction expressions for the subgroup properties
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The property of being fully invariant is the invariance property with respect to endomorphisms. It has the function restriction expression:

Endomorphism $\to$ Function

The property of being characteristic is the invariance property with respect to automorphisms. It has the function restriction expression:

Automorphism $\to$ Function

Since the left side of the function restriction expression for characteristicity is stronger while the right sides of both function restriction expressions are equal, the property of being characteristic is weaker than the property of being fully invariant.