Paranormal subgroup

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
This article is about a definition in group theory that is standard among the group theory community (or sub-community that dabbles in such things) but is not very basic or common for people outside.
VIEW: Definitions built on this | Facts about this: (facts closely related to Paranormal subgroup, all facts related to Paranormal subgroup) |Survey articles about this | Survey articles about definitions built on this
VIEW RELATED: Analogues of this | Variations of this | Opposites of this |
View a list of other standard non-basic definitions
This article defines a subgroup property: a property that can be evaluated to true/false given a group and a subgroup thereof, invariant under subgroup equivalence. View a complete list of subgroup properties[SHOW MORE]
This is a variation of normality|Find other variations of normality | Read a survey article on varying normality

Definition

Definition with symbols

(right-action convention): A subgroup $H$ of a group $G$ is termed paranormal if for any $g \in G$, $H$ is a contranormal subgroup of $\langle H, H^g \rangle$; in other words, the normal closure of $H$ in $\langle H, H^g \rangle$ is $\langle H, H^g \rangle$.

Here $H^g = g^{-1}Hg$ is a conjugate of $H$, and the angled braces are for the subgroup generated.

(left-action convention): A subgroup $H$ of a group $G$ is termed paranormal if for any $g \in G$, $H$ is a contranormal subgroup of $\langle H, gHg^{-1} \rangle$; in other words, the normal closure of $H$ in $\langle H, gHg^{-1} \rangle$ is $\langle H, gHg^{-1} \rangle$.

Metaproperties

Intermediate subgroup condition

YES: This subgroup property satisfies the intermediate subgroup condition: if a subgroup has the property in the whole group, it has the property in every intermediate subgroup.
ABOUT THIS PROPERTY: View variations of this property satisfying intermediate subgroup condition | View variations of this property not satisfying intermediate subgroup condition
ABOUT INTERMEDIATE SUBROUP CONDITION:View all properties satisfying intermediate subgroup condition | View facts about intermediate subgroup condition

Trimness

This subgroup property is trim -- it is both trivially true (true for the trivial subgroup) and identity-true (true for a group as a subgroup of itself).
View other trim subgroup properties | View other trivially true subgroup properties | View other identity-true subgroup properties

Join-closedness

YES: This subgroup property is join-closed: an arbitrary (nonempty) join of subgroups with this property, also has this property.
ABOUT THIS PROPERTY: View variations of this property that are join-closed | View variations of this property that are not join-closed
ABOUT JOIN-CLOSEDNESS: View all join-closed subgroup properties (or, strongly join-closed properties) | View all subgroup properties that are not join-closed | Read a survey article on proving join-closedness | Read a survey article on disproving join-closedness

In fact, an arbitrary, possibly empty, join of paranormal subgroups is paranormal. For full proof, refer: Paranormality is strongly join-closed

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: IsParanormal
View other GAP-codable subgroup properties | View subgroup properties with in-built commands
GAP-codable subgroup property

There is no built-in GAP command to test paranormality, but a short piece of GAP code can achieve this. The code is available at GAP:IsParanormal.