Retract

Definition

Equivalent definitions in tabular format

No.	Shorthand	A subgroup of a group is termed a retract if ...	A subgroup $H$ of a group $G$ is termed a retract of $G$ if ...
1	image of idempotent endomorphism	there is an idempotent endomorphism of the group whose image is precisely that subgroup. This idempotent endomorphism is termed the retraction.	there is an endomorphism $\sigma$ of $G$ such that $\sigma ^{2}=\sigma$ and the image of $\sigma$ is precisely $H$.
2	normal complement	it has a normal complement: a normal subgroup that intersects it trivially, and that together with it generates the whole group.	there is a normal subgroup $N$ of $G$ such that $NH=G$ and $N\cap H$ is trivial.
3	homomorphism extension	any homomorphism from the subgroup to any group extends to a homomorphism from the whole group to that group	for any homomorphism of groups $\varphi :H\to K$ to any group $K$, there exists a homomorphism $\theta :G\to K$ such that $\theta |_{H}=\varphi$.

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View other semistandard definitions in group theory

History

Introduction of the concept

The concept of retract is fairly old, and came about in the beginning of the study of group theory. Retracts were first encountered as the right part in short exact sequences that split.

Introduction of the term

The term retract is not very standard, and the concept is often referred to without the use of this formal term. The term retract actually comes from the set-theoretic/topological equivalent notion.

Further information: Retractions and functors

Formalisms

BEWARE! This section of the article uses terminology local to the wiki, possibly without giving a full explanation of the terminology used (though efforts have been made to clarify terminology as much as possible within the particular context)

Monadic second-order description

This subgroup property is a monadic second-order subgroup property, viz., it has a monadic second-order description in the theory of groups
View other monadic second-order subgroup properties

Relation with other properties

Stronger properties

Property	Meaning	Proof of implication	Proof of strictness (reverse implication failure)	Intermediate notions
direct factor		direct factor implies retract	retract not implies direct factor	|FULL LIST, MORE INFO
free factor		free factor implies retract	retract not implies free factor	|FULL LIST, MORE INFO
regular retract				|FULL LIST, MORE INFO

Weaker properties

Property	Meaning	Proof of implication	Proof of strictness (reverse implication failure)	Intermediate notions
endomorphism image	image under an endomorphism (not necessarily idempotent) of the whole group			|FULL LIST, MORE INFO
conjugation-invariantly permutably complemented subgroup	there is a permutable complement to it that is also a permutable complement to all its conjugate subgroups			|FULL LIST, MORE INFO
permutably complemented subgroup	there is a permutable complement to it			|FULL LIST, MORE INFO
lattice-complemented subgroup	there is a lattice complement to it			|FULL LIST, MORE INFO
subset-conjugacy-closed subgroup	any conjugation between two subsets in the whole group can also be achieved by conjugation in the subgroup	retract implies subset-conjugacy-closed	subset-conjugacy-closed not implies retract	|FULL LIST, MORE INFO
conjugacy-closed subgroup	any two elements that are conjugate in the whole group are conjugate in the subgroup	retract implies conjugacy-closed	(via subset-conjugacy-closed)	|FULL LIST, MORE INFO
central factor of normalizer		retract implies WC		|FULL LIST, MORE INFO
c-normal subgroup				|FULL LIST, MORE INFO
verbally closed subgroup				|FULL LIST, MORE INFO
subgroup whose derived subgroup equals its intersection with whole derived subgroup		Retract implies derived subgroup equals intersection with whole derived subgroup
local divisibility-closed subgroup	if an element in the subgroup has a $n^{th}$ root in the whole group, it has a $n^{th}$ root in the subgroup.	(via verbally closed)	(via verbally closed)	|FULL LIST, MORE INFO
local powering-invariant subgroup	if an element in the subgroup has a unique $n^{th}$ root in the whole group, that root is in the subgroup.	(via local divisibility-closed)	(via local divisibility-closed)	|FULL LIST, MORE INFO
divisibility-closed subgroup	if every element in the subgroup has a $n^{th}$ root in the whole group, every element has a $n^{th}$ root in the subgroup.			|FULL LIST, MORE INFO
powering-invariant subgroup	if every element has a unique $n^{th}$ root in the group, every element of the subgroup has a unique $n^{th}$ root in the subgroup.	(via local divisibility-closed)	(via local divisibility-closed)	|FULL LIST, MORE INFO

Related properties

Property	Meaning	Proof of one non-implication	Proof of other non-implication	Notions stronger than both	Notions weaker than both
normal subgroup	invariant under all inner automorphisms	retract not implies normal	normal not implies direct factor	direct factor is the conjunction	|FULL LIST, MORE INFO

Metaproperties

BEWARE! This section of the article uses terminology local to the wiki, possibly without giving a full explanation of the terminology used (though efforts have been made to clarify terminology as much as possible within the particular context)

Metaproperty name	Satisfied?	Proof	Statement with symbols
transitive subgroup property	Yes	retract is transitive	If $H\leq K\leq G$ are groups such that $H$ is a retract of $K$ and $K$ is a retract of $G$, then $H$ is a retract of $G$.
trim subgroup property	Yes		In any group, the whole group and the trivial subgroup are retracts.
finite-intersection-closed subgroup property	No		It is possible to have a group $G$ and subgroups $H$ and $K$ of $G$ such that both $H$ and $K$ are retracts but $H\cap K$ is not a retract of $G$.

Testing

GAP code

One can write code to test this subgroup property in GAP (Groups, Algorithms and Programming), though there is no direct command for it.
View the GAP code for testing this subgroup property at: IsRetract
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GAP-codable subgroup property