Subgroup-quotient duality for groups

There are many loose forms of subgroup-quotient duality, i.e., ways of transforming a definition or fact that is about subgroups to a similar fact about quotients, or more generally, ways of interchanging the roles played by subgroups and quotients in an existing definition. One way of thinking about this is that we are trying to "reverse the arrows" or go to the "opposite category" in a crude sense.

The quick glossary of this duality is given below:

Quick correspondence	Caveats and explanation
subgroup $\leftrightarrow$ quotient group or normal subgroup $\leftrightarrow$ quotient group	The idea is that instead of quantifying statements over subgroups, we quantify them over quotient groups.
injectivity $\leftrightarrow$ surjectivity	The idea here is that the way injectivity interacts with subgroups is similar to the way surjectivity interacts with quotients. Similarly, the way injectivity interacts with quotients is similar to the way surjectivity interacts with subgroups. For instance, an injective map (not necessarily an endomorphism) from a group to itself restricts to a injective map on any subgroup to which its restriction is well-defined. However, this is not necessary for surjective maps. On the other hand, a surjective map, whenever it descends to a quotient, descends to a surjective map on the quotient.
image of internal map (not necessarily homomorphism) $\leftrightarrow$ kernel of internal map	We will see this more clearly explained in the verbal-marginal duality case.

Situations where the duality is rigorous

The main situation where the duality is rigorous is the case of a finite abelian group. See subgroup lattice and quotient lattice of finite abelian group are isomorphic.

Subgroup properties

Below are listed some very basic subgroup properties along with their duals:

Subgroup property	Meaning	Dual property is the property of being a quotient group by what kind of subgroup?	Meaning
normal subgroup	invariant under all inner automorphisms	normal subgroup	the quotient group is invariant under inner automorphisms.
characteristic subgroup	invariant under all automorphisms	characteristic subgroup	the quotient group is invariant under all automorphisms.
injective endomorphism-invariant subgroup	invariant under all injective endomorphisms	strictly characteristic subgroup	the subgroup is invariant under all surjective endomorphisms, so these descend to the quotient group.
fully invariant subgroup	invariant under all endomorphisms	fully invariant subgroup	invariant under all endomorphisms
isomorph-free subgroup	no other isomorphic subgroups	quotient-isomorph-free subgroup	no other subgroup with isomorphic quotient group.
isomorph-containing subgroup	any isomorphic subgroup is contained in it.	quotient-isomorph-containing subgroup	contained in any subgroup with an isomorphic quotient group.
homomorph-containing subgroup	contains any homomorphic image of itself.	quotient-homomorph-containing subgroup	any homomorphic image of the quotient group is the quotient group by a subgroup containing it.
verbal subgroup	image of a word map	marginal subgroup	elements that don't affect the word map (sort of like the kernel).
direct factor	factor in an internal direct product	direct factor	factor in an internal direct product
retract	subgroup with a normal complement	complemented normal subgroup	normal subgroup with a permutable complement.

Basic verbal-marginal duality

The verbal-marginal duality is a loose, not very precise duality between statements that can be made about verbal subgroups and their factor groups versus marginal subgroups and their factor groups. The duality is difficult to work with rigorously, but it helps as an intuitive guide. The rough idea is that verbal subgroups start constructively by looking at images of word maps, so that the quotient groups are nice and "small" while marginal subgroups start by solving equations, so it is the subgroups themselves that are nice and "small."

Parameter/specification	Notion on the verbal side	Notion on the marginal side	Explanation of duality
a collection of words	verbal subgroup for the collection of words	the quotient group corresponding to the marginal subgroup for the collection of words.	the verbal subgroup is the image of the word map. The marginal subgroup is like the coordinate-wise kernel of the word map, so the quotient group by it should be qualitatively similar. Combining these pieces of data gives a notion of isologism of groups.
a collection of words	quotient group by the verbal subgroup for the collection of words	the marginal subgroup for the collection of words	both these groups are in the variety of groups where the words are true. The quotient group is the largest such quotient. The subgroup is not the largest such subgroup, however.
--	derived subgroup	inner automorphism group	if you take the collection of all words obtained by taking products of commutators, the derived subgroup is the corresponding verbal subgroup and the inner automorphism group is the quotient group by the corresponding marginal subgroup. Combining these pieces of data gives a notion of isoclinism of groups.
--	abelianization	center	if you take the collection of all words obtained by taking products of commutators, the abelianization is the verbal quotient group and the center is the marginal subgroup.

Group properties

Property	Meaning	Dual property	Meaning
perfect group	equals its own derived subgroup abelianization is trivial	centerless group	the center is trivial, so the inner automorphism group is the whole group.
Schur-trivial group		group in which every automorphism is inner
superperfect group	Schur-trivial and perfect	complete group	centerless and every automorphism is inner.

Group metaproperties

Property	Meaning	Dual property	Meaning
subgroup-closed group property	if a group has the property, so does every subgroup.	quotient-closed group property	if a group has the property, so does every quotient group.
normal subgroup-closed group property	if a group has the property, so does every normal subgroup.	quotient-closed group property	if a group has the property, so does every quotient group.
characteristic subgroup-closed group property	if a group has the property, so does every characteristic subgroup	characteristic quotient-closed group property	if a group has the property, so does every quotient group by a characteristic subgroup.

Subgroup metaproperties

Metaproperty	Meaning: A subgroup property $p$ satisfies this metaproperty if ...	Dual metaproperty (translated into subgroup property terms from quotient property terms)	Meaning: A subgroup property $p$ satisfies this metaproperty if ...
transitive subgroup property	whenever $H\leq K\leq G$ are such that $H$ satisfies $p$ in $K$ and $K$ satisfies $p$ in $G$, then $H$ satisfies $p$ in $G$.	quotient-transitive subgroup property	whenever $H\leq K\leq G$ are such that $H$ is normal and satisfies $p$ in $G$ and further, $K/H$ satisfies $p$ in $G/H$, then $K$ satisfies $p$ in $G$.
intermediate subgroup condition	whenever $H\leq K\leq G$ are such that $H$ satisfies $p$ in $G$, then $H$ also satisfies $p$ in $K$.	image condition	whenever $H\leq G$ is such that $H$ satisfies $p$ in $G$, and $\varphi :G\to K$ is a surjective homomorphism, $\varphi (H)$ satisfies $p$ in $K$.
trivially true subgroup property	in any group $G$, the trivial subgroup satisfies $p$.	identity-true subgroup property	in any group $G$, $G$ satisfies $p$ as a subgroup of itself.