Upward-closed normal subgroup
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 property of a normal subgroup is completely characterized by the abstract isomorphism class of the quotient group
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- Every subgroup intermediate between the given subgroup and the whole group, is normal in the whole group.
- It is normal and the quotient group is a Dedekind group, viz all subgroups of the quotient group are normal in it.
Definition with symbols
A subgroup of a group is termed upward-closed normal if it satisfies the following equivalent conditions:
- For any subgroup such that , .
- , and the quotient group is a Dedekind group, viz., all its subgroups are normal.
In terms of the upward closure operator
Relation with other properties
NO: This subgroup property is not transitive: a subgroup with this property in a subgroup with this property, need not have the property in the whole group
ABOUT THIS PROPERTY: View variations of this property that are transitive|View variations of this property that are not transitive
ABOUT TRANSITIVITY: View a complete list of subgroup properties that are not transitive|View facts related to transitivity of subgroup properties | View a survey article on disproving transitivity
Clearly, an upward-closed normal subgroup of an upward-closed normal subgroup need not be normal. In fact, in any solvable non-Abelian group, the second derived subgroup is an upward-closed normal subgroup of an upward-closed normal subgroup, yet it is usually not itself upward-closed normal.
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
The property of being upward-closed normal satisfies the intermediate subgroup condition, viz if and is upward-closed normal in , is also upward-closed normal in . This follows essentially from the fact that normality satisfies the intermediate subgroup condition.
The property of being upward-closed normal is not trivially true. However, it is an identity-true subgroup property, viz every group is upward-closed normal as a subgroup of itself.