Subnormal subgroup: Difference between revisions

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{{semibasicdef}}
{{subgroup property}}
{{subgroup property}}
 
{{variation of|normality}}
{{semibasicdef}}
[[importance rank::2| ]]
 
{{variationof|normality}}
==Definition==
==Definition==


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A [[subgroup]] of a [[group]] is termed '''subnormal''' if any of the following equivalent conditions holds:
A [[subgroup]] of a [[group]] is termed '''subnormal''' if any of the following equivalent conditions holds:


* There is a finite ascending chain of subgroups starting from the subgroup and going till the whole group, such that each is a normal subgroup of its successor. The smallest possible length of such a chain is termed the [[subnormal depth]] of the subnormal subgroup.
# There is a finite ascending chain of subgroups starting from the subgroup and going till the whole group, such that each is a [[defining ingredient::normal subgroup]] of its successor. The smallest possible length of such a chain is termed the [[subnormal depth]] of the subnormal subgroup.
* Consider the descending chain of subgroups defined as follows: each member is the [[normal closure]] of the original subgroup in its predecessor. This descending chain must reach the original subgroup within finitely many steps. The number of steps it takes is termed the subnormal depth.
# Consider the descending chain of subgroups defined as follows: each member is the [[normal closure]] of the original subgroup in its predecessor. This descending chain must reach the original subgroup within finitely many steps. The number of steps it takes is termed the subnormal depth.
* The sequence of subgroups starting with the whole group, and where each is the commutator of its predecessor with the subgroup, gets inside the given subgroup after finitely many steps. The number of steps is takes is termed the subnormal depth.
# The sequence of subgroups starting with the whole group, and where each is the [[commutator of two subgroups|commutator]] of its predecessor with the subgroup, gets inside the given subgroup after finitely many steps. The number of steps is takes is termed the subnormal depth.


The equivalence of the definitions thus must also show that the notions of ''subnormal depth'' in each case are the same.
The equivalence of the definitions thus must also show that the notions of ''subnormal depth'' in each case are the same.
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A subgroup <math>H</math> is termed '''subnormal''' in a group <math>G</math> if either of the following equivalent conditions holds:
A subgroup <math>H</math> is termed '''subnormal''' in a group <math>G</math> if either of the following equivalent conditions holds:


* There exists an ascending chain <math>H = H_0</math> &le; <math>H_1</math> ...<math>H_n = G</math> such that each <math>H_i</math> is normal in <math>H_{i+1}</math>. The smallest possible <math>n</math> for which such a chain exists is termed the subnormal depth of <math>H</math>.
# There exists an ascending chain <math>H = H_0 \le H_1 \le \dots \le H_n = G</math> such that each <math>H_i</math> is normal in <math>H_{i+1}</math>. The smallest possible <math>n</math> for which such a chain exists is termed the subnormal depth of <math>H</math>.
* Consider the descending chain <math>G_i</math> defined as follows: <math>G_0 = G</math> and <math>G_{i+1}</math> is the [[normal closure]] of <math>H</math> in <math>G_i</math>. Then, there exists an <math>n</math> for which <math>G_n = H</math>. The smallest such <math>n</math> is termed the subnormal depth of <math>H</math>.
# Consider the descending chain <math>G_i</math> defined as follows: <math>G_0 = G</math> and <math>G_{i+1}</math> is the [[normal closure]] of <math>H</math> in <math>G_i</math>. Then, there exists an <math>n</math> for which <math>G_n = H</math>. The smallest such <math>n</math> is termed the subnormal depth of <math>H</math>.
* Consider the sequence <math>K_i</math> of subgroups of <math>G</math> defined as follows: <math>K_0 = G</math>, and <math>K_{i+1} = [H,K_i]</math>, eventually enters inside <math>H</math>. The number of steps taken is termed the '''subnormal depth''' of <math>H</math>.
# Consider the sequence <math>K_i</math> of subgroups of <math>G</math> defined as follows: <math>K_0 = G</math>, and <math>K_{i+1} = [H,K_i]</math> (the [[commutator of two subgroups|commutator]]), This sequence of subgroups eventually enters inside <math>H</math>. The number of steps taken is termed the '''subnormal depth''' of <math>H</math>.
 
A <math>k</math>-subnormal subgroup is a subnormal subgroup with subnormal depth at most <math>k</math>.


===Equivalence of definitions===
===Equivalence of definitions===


{{proofat|[[Equivalence of definitions of subnormal subgroup]]}}
{{proofat|[[Equivalence of definitions of subnormal subgroup]]}}
==Examples==
{{subgroup property see examples}}


==Formalisms==
==Formalisms==
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The property of being a subnormal subgroup is obtained by applying the [[subordination operator]] to the group property of [[normality]].
The property of being a subnormal subgroup is obtained by applying the [[subordination operator]] to the group property of [[normality]].
==Facts==
* [[There exist subgroups of arbitrarily large subnormal depth]]: For any positive integer <math>k</math>, there exists a group <math>G</math> and a subgroup <math>H</math> of <math>G</math> such that the subnormal depth of <math>H</math> in <math>G</math> is precisely <math>k</math>..
==Metaproperties==
{{wikilocal-section}}
{| class="sortable" border="1"
! Metaproperty name !! Satisfied? !! Proof !! Statement with symbols
|-
| [[Satisfies metaproperty::transitive subgroup property]] || Yes || (by definition) || If <math>H \le K \le G</math>, with <math>H</math> subnormal in <math>K</math> and <math>K</math> subnormal in <math>G</math>, then <math>H</math> is subnormal in <math>G</math>.
|-
| [[Satisfies metaproperty::trim subgroup property]] || Yes || (by definition) || The trivial subgroup and the whole group are subnormal in the group.
|-
| [[Satisfies metaproperty::finite-intersection-closed subgroup property]] || Yes || [[Subnormality is finite-intersection-closed]] || If <math>H_1, H_2</math> are subnormal subgroups of <math>G</math>, then <math>H_1 \cap H_2</math> is also subnormal in <math>G</math>.
|-
| [[Satisfies metaproperty::finite-relative-intersection-closed subgroup property]]  || Yes || [[Subnormality is finite-relative-intersection-closed]] || In other words, if <math>H, K \le G</math> are subgroups such that <math>H</math> is <math>h</math>-subnormal in <math>G</math> and <math>K</math> is <math>k</math>-subnormal in some subgroup <math>L</math> of <math>G</math> containing both <math>H</math> and <math>K</math>, then <math>H \cap K</math> is <math>(h + k)</math>-subnormal in <math>G</math>.
|-
| [[Satisfies metaproperty::intermediate subgroup condition]] || Yes || [[Subnormality satisfies intermediate subgroup condition]] || If <math>H \le K \le G</math> with <math>H</math> subnormal in <math>G</math>, then <math>H</math> is subnormal in <math>K</math>.
|-
| [[Dissatisfies metaproperty::finite-join-closed subgroup property]] || No || [[Subnormality is not finite-join-closed]] || It is possible to have subnormal subgroups <math>H,K</math> of a group <math>G</math> such that <math>\langle H, K \rangle</math> is not subnormal in <math>G</math>.
|-
| [[Satisfies metaproperty::normalizing join-closed subgroup property]] || Yes || [[Subnormality is normalizing join-closed]] || If <math>H, K \le G</math> are both subnormal subgroups and <math>K \le N_G(H)</math>, then <math>HK</math> is also a subnormal subgroup.
|-
| [[Satisfies metaproperty::permuting join-closed subgroup property]] || Yes || [[Subnormality is permuting join-closed]] || If <math>H, K \le G</math> are both subnormal subgroups and <math>HK = KH</math>, then <math>HK</math> is also a subnormal subgroup.
|-
| [[Satisfies metaproperty::transfer condition]] || Yes || [[Subnormality satisfies transfer condition]] || If <math>H,K \le G</math> with <math>H</math> subnormal, then <math>H \cap K</math> is subnormal in <math>K</math>.
|-
| [[Dissatisfies metaproperty::centralizer-closed subgroup property]] || No || [[Subnormality is not centralizer-closed]] || It is possible to have <math>H \le G</math> subnormal such that <math>C_G(H)</math> is not subnormal in <math>G</math>.
|-
| [[Dissatisfies metaproperty::commutator-closed subgroup property]] || No || [[Subnormality is not commutator-closed]] || It is possible to have subnormal subgroups <math>H, K \le G</math> such that the [[commutator of two subgroups|commutator]] <math>[H,K]</math> is not subnormal in <math>G</math>.
|-
| [[Dissatisfies metaproperty::finite-upper join-closed subgroup property]] || No || [[Subnormality is not finite-upper join-closed]] || It is possible to have <math>H \le G</math> with intermediate subgroups <math>K_1, K_2</math> such that <math>H</math> is subnormal in <math>K_1</math> and in <math>K_2</math> but not in <math>\langle K_1, K_2 \rangle</math>.
|-
| [[dissatisfies metaproperty::conditionally lattice-determined subgroup property]] || No || [[No subgroup property between normal Sylow and subnormal or between Sylow retract and retract is conditionally lattice-determined]] || It is possible to have a group <math>G</math>, an automorphism <math>\varphi</math> of the [[lattice of subgroups]], and a subnormal subgroup <math>H</math> of <math>G</math> such that <math>\varphi(H)</math> is not subnormal.
|}


==Relation with other properties==
==Relation with other properties==
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===Stronger properties===
===Stronger properties===


* [[Normal subgroup]]: The property of being normal is the same as that of being <math>1</math>-subnormal.
{| class="sortable" border="1"
* [[Finitarily hypernormalized subgroup]]: {{proofat|[[Finitarily hypernormalized implies subnormal]]}}
! Property !! Meaning !! Proof of implication !! Proof of strictness (reverse implication failure) !! Intermediate notions
* [[2-subnormal subgroup]]
|-
* [[Permutable subgroup]] and [[Conjugate-permutable subgroup]] for the case of finite groups: {{proofat|[[Conjugate-permutable implies subnormal (finite groups)]]}}
| [[Weaker than::Normal subgroup]] || subnormal of depth 1 || (obvious) || [[subnormal not implies normal]] (also see [[normality is not transitive]], [[there exist subgroups of arbitrarily large subnormal depth]]) || {{intermediate notions short|subnormal subgroup|normal subgroup}}
|-
|[[Weaker than::Finitarily hypernormalized subgroup]] || taking the normalizer finitely many times yields the whole group || || follows from [[abnormal normalizer and 2-subnormal not implies normal]] || {{intermediate notions short|subnormal subgroup|finitarily hypernormalized subgroup}}
|-
| [[Weaker than::2-subnormal subgroup]] || normal subgroup of normal subgroup || (obvious) || [[there exist subgroups of arbitrarily large subnormal depth]] || {{intermediate notions short|subnormal subgroup|2-subnormal subgroup}}
|-
| [[Weaker than::3-subnormal subgroup]] || normal subgroup of normal subgroup of normal subgroup || (obvious) || [[there exist subgroups of arbitrarily large subnormal depth]] || {{intermediate notions short|3-subnormal subgroup|subnormal subgroup}}
|-
| [[Weaker than::4-subnormal subgroup]] || normal of normal of normal of normal || (obvious) || [[there exist subgroups of arbitrarily large subnormal depth]] || {{intermediate notions short|4-subnormal subgroup|subnormal subgroup}}
|-
| [[Weaker than::Join-transitively subnormal subgroup]] || join with any subnormal subgroup is subnormal || because the trivial subgroup is subnormal || [[subnormality is not finite-join-closed]] || {{intermediate notions short|subnormal subgroup|join-transitively subnormal subgroup}}
|-
| [[Weaker than::Left-transitively fixed-depth subnormal subgroup]] || || || [[subnormal not implies left-transitively fixed-depth subnormal]] || {{intermediate notions short|subnormal subgroup|left-transitively fixed-depth subnormal subgroup}}
|-
| [[Weaker than::Right-transitively fixed-depth subnormal subgroup]] || || || [[subnormal not implies right-transitively fixed-depth subnormal]] || {{intermediate notions short|subnormal subgroup|right-transitively fixed-depth subnormal subgroup}}
|-
| [[Weaker than::Linear-bound join-transitively subnormal subgroup]] ||join with a <math>k</math>-subnormal subgroup is subnormal of depth bounded by a linear function of <math>k</math> || || ||
|-
| [[Weaker than::Polynomial-bound join-transitively subnormal subgroup]] || join with a <math>k</math>-subnormal subgroup is subnormal of depth bounded by a polynomial function of <math>k</math> || || ||
|-
| [[Weaker than::Subnormal subgroup of finite group]] || || || ||
|-
| [[Weaker than::Permutable subgroup of finite group]] || || [[permutable implies subnormal in finite]] || ||
|-
| [[Weaker than::Conjugate-permutable subgroup of finite group]] || || [[conjugate-permutable implies subnormal in finite]] || ||
|}


===Weaker properties===
===Weaker properties===


* [[Ascendant subgroup]]
{| class="sortable" border="1"
* [[Descendant subgroup]]
! Property !! Meaning !! Proof of implication !! Proof of strictness (reverse implication failure) !! Intermediate notions
* [[Serial subgroup]]
|-
* [[Subpronormal subgroup]]
| [[Stronger than::Ascendant subgroup]] || ascending (possibly transfinite) chain from subgroup to whole group, each normal in successor || [[subnormal implies ascendant]] || [[ascendant not implies subnormal]] || {{intermediate notions short|ascendant subgroup|subnormal subgroup}}
|-
| [[Stronger than::Descendant subgroup]] ||  ||[[subnormal implies descendant]]|| [[descendant not implies subnormal]] || {{intermediate notions short|descendant subgroup|subnormal subgroup}}
|-
| [[Stronger than::Serial subgroup]] || || (via ascendant, descendant) || (via ascendant, descendant) || {{intermediate notions short|serial subgroup|subnormal subgroup}}
|-
| [[Stronger than::Subpronormal subgroup]] || || || || {{intermediate notions short|subpronormal subgroup|subnormal subgroup}}
|-
| [[Stronger than::Locally subnormal subgroup]] || subnormal in its join with any finitely generated subgroup || || || {{intermediate notions short|locally subnormal subgroup|subnormal subgroup}}
|-
| [[Stronger than::Almost subnormal subgroup]] || ascending chain from subgroup to whole group with each member either normal or of finite index in successor || || || {{intermediate notions short|almost subnormal subgroup|subnormal subgroup}}
|-
| [[Stronger than::Join of finitely many subnormal subgroups]] || join of finitely many subnormal subgroups || || ||
|-
| [[Stronger than::Join of subnormal subgroups]] || join of subnormal subgroups || || ||
|-
| [[Stronger than::Upper join of subnormal subgroups]] || the whole group is a join of intermediate subgroups in each of which the given subgroup is subnormal. || || ||
|}


===Conjunction with other properties===
===Conjunction with other properties===
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* Conjunction with [[subnormal-to-normal subgroup]] gives [[normal subgroup]]
* Conjunction with [[subnormal-to-normal subgroup]] gives [[normal subgroup]]
* Conjunction with [[pronormal subgroup]] gives [[normal subgroup]] (this generalizes, in fact, to any property that is stronger than being subnormal-to-normal): {{proofat|[[Pronormal and subnormal implies normal]]}}
* Conjunction with [[pronormal subgroup]] gives [[normal subgroup]] (this generalizes, in fact, to any property that is stronger than being subnormal-to-normal): {{proofat|[[Pronormal and subnormal implies normal]]}}
In fact, there are a large number of subgroup properties whose conjunction with subnormality gives normality. {{further|[[subnormal-to-normal and normal-to-characteristic]]}}
Conjunction with group properties:
* [[Abelian subnormal subgroup]] is a subnormal subgroup that is also an [[abelian group]]. Also related:
** [[Nilpotent subnormal subgroup]] is a subnormal subgroup that is also a [[nilpotent group]].
** [[Cyclic subnormal subgroup]] is a subnormal subgroup that is also a [[cyclic group]].
** [[Solvable subnormal subgroup]] is a subnormal subgroup that is also a [[solvable group]].
* [[Finite subnormal subgroup]] is a subnormal subgroup that is also a [[finite group]].
* [[Perfect subnormal subgroup]] is a subnormal subgroup that is also a [[perfect group]]. Also related:
** [[Simple subnormal subgroup]] is a subnormal subgroup that is also a [[simple group]].
** [[Component]] is a subnormal subgroup that is also a [[quasisimple group]].
The property of being subnormal in particular kinds of groups is also of interest:
* [[Subgroup of nilpotent group]] is a subnormal subgroup of a [[nilpotent group]]. (Note that [[nilpotent implies every subgroup is subnormal]]).
* [[Subnormal subgroup of finite group]] is a subnormal subgroup of a [[finite group]].
* [[Subnormal subgroup of solvable group]] is a subnormal subgroup of a [[solvable group]].


===Related group properties===
===Related group properties===


* [[Group in which every subgroup is subnormal]] is a group in which every subgroup is subnormal. [[Nilpotent group]]s have this property: [[nilpotent implies every subgroup is subnormal]].
* [[Subnormal intersection property]] is the property of being a group where an arbitrary intersection of subnormal subgroups is subnormal.
* [[Subnormal intersection property]] is the property of being a group where an arbitrary intersection of subnormal subgroups is subnormal.
* [[Group satisfying subnormal join property|Subnormal join property]] is the property of being a group where a join of finitely many subnormal subgroups is subnormal.
* [[Group satisfying generalized subnormal join property|Generalized subnormal join property]] is the property of being a group where an arbitrary join of subnormal subgroups is subnormal.
* [[T-group]] is the property of being a group in which every subnormal subgroup is [[normal subgroup|normal]].
* [[T-group]] is the property of being a group in which every subnormal subgroup is [[normal subgroup|normal]].


==Metaproperties==
==Effect of property operators==


{{transitive}}
{{wikilocal-section}}


The property of being a subnormal subgroup is a [[transitive subgroup property]]. That is, any subnormal subgroup of a subnormal subgroup is subnormal.
{| class="wikitable" border="1"
! Operator !! Meaning !! Result of application !! Proof
|-
| [[left transiter]] || Whenever the whole group is subnormal in some bigger group, so is the subgroup. || subnormal subgroup || subnormality is a [[t.i. subgroup property]], so equals its own left transiter.
|-
| [[right transiter]] || Any subnormal subgroup of the subgroup is subnormal in the whole group. || subnormal subgroup || subnormality is a [[t.i. subgroup property]], so equals its own right transiter.
|-
| [[subordination operator]] || There is a finite series of subgroups from the subgroup to the whole group with each subnormal in the next. || subnormal subgroup || subnormality is a [[t.i. subgroup property]], so equals its own subordination.
|-
| [[join-transiter]] || The join with any subnormal subgroup is subnormal. || [[join-transitively subnormal subgroup]] || (by definition)
|-
| [[finite-join-closure operator]] || The join in the whole group of finitely many subnormal subgroups. || [[join of finitely many subnormal subgroups]] || (by definition)
|-
| [[join-closure operator]] || The join in the whole group of subnormal subgroups. || [[join of subnormal subgroups]] || (by definition)
|-
| [[intersection-closure operator]] || The intersection in the whole group of subnormal subgroups || [[intersection of subnormal subgroups]] || (by definition)
|}


{{trim}}
More on these property operators: <toggledisplay>
{{applyingoperatorgives|left transiter|subnormal subgroup}}


The property of being subnormal is [[trivially true subgroup property|trivially true]], that is, the trivial subgroup is always subnormal.
Since subnormality is a [[t.i. subgroup property]], it equals its own left transiter. However, there is a stronger property of being a [[left-transitively fixed-depth subnormal subgroup]].


The property of being subnormal is also [[identity-true subgroup property|identity-true]], that is,  every group is subnormal as a subgroup of itself.
{{applyingoperatorgives|right transiter|subnormal subgroup}}


{{finite-intersection-closed}}
Since subnormality is a [[t.i. subgroup property]], it equals its own right transiter. However, there is a stronger property of being a [[right-transitively fixed-depth subnormal subgroup]].


The property of being subnormal is a [[finite-intersection-closed subgroup property]]. That is, any finite intersection of subnormal subgroups is a subnormal subgroups.
{{applyingoperatorgives|join-transiter|join-transitively subnormal subgroup}}


{{intsubcondn}}
A subgroup <math>H \le G</math> is termed join-transitively subnormal if <math>\langle H, K \rangle</math> is subnormal for any subnormal subgroup <math>K</math>. Any [[normal subgroup]], [[2-subnormal subgroup]] or [[permutable subnormal subgroup]] is join-transitively subnormal.


The property of subnormality satisfies [[intermediate subgroup condition]]. That is, if <math>H</math> is a subnormal subgroup of <math>G</math>, and <math>K</math> is an intermediate subgroup of <math>G</math> containing <math>H</math>, then <math>H</math> is a subnormal subgroup of <math>K</math>.
{{applyingoperatorgives|finite-join-closure operator|join of finitely many subnormal subgroups}}


{{join-closed}}
{{applyingoperatorgives|join-closure operator|join of subnormal subgroups}}


{{transfercondn}}
{{applyingoperatorgives|intersection-closure operator|intersection of subnormal subgroups}}
</toggledisplay>


==Testing==
==Testing==
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The property of being subnormal can be tested for permutation groups. In fact, it can be tested if we have an algorithm for [[normal closure-finding]].
The property of being subnormal can be tested for permutation groups. In fact, it can be tested if we have an algorithm for [[normal closure-finding]].


{{GAP command for sgp|IsSubnormal}}
{{GAP command for subgroup property|
test = IsSubnormal}}


The GAP syntax for determining whether a subgroup is subnormal is:
The GAP syntax for determining whether a subgroup is subnormal is:
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The class 20D35 studies subnormal subgroups for finite groups. In the infinite case, 20E15 is more appropriate.
The class 20D35 studies subnormal subgroups for finite groups. In the infinite case, 20E15 is more appropriate.
==References==
===Textbook references===
* {{booklink|RobinsonGT}}, Page 63 (definition introduced in the context of a more general definition)
* {{booklink|AlperinBell}}, Page 92-93 (introduces notion of subnormal series, does not explicitly talk of ''subnormal subgroup'')
* {{booklink|FGTAsch}}, Page 23 (formal definition)
* {{booklink|Gorenstein}}, Page 13, Chapter 1, Exercise 5 (definition introduced in exercise)


==External links==
==External links==

Latest revision as of 15:40, 16 April 2017

This article is about a standard (though not very rudimentary) definition in group theory. The article text may, however, contain more than just the basic definition
VIEW: Definitions built on this | Facts about this: (facts closely related to Subnormal subgroup, all facts related to Subnormal 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 complete list of semi-basic definitions on this wiki

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

Symbol-free definition

A subgroup of a group is termed subnormal if any of the following equivalent conditions holds:

  1. There is a finite ascending chain of subgroups starting from the subgroup and going till the whole group, such that each is a normal subgroup of its successor. The smallest possible length of such a chain is termed the subnormal depth of the subnormal subgroup.
  2. Consider the descending chain of subgroups defined as follows: each member is the normal closure of the original subgroup in its predecessor. This descending chain must reach the original subgroup within finitely many steps. The number of steps it takes is termed the subnormal depth.
  3. The sequence of subgroups starting with the whole group, and where each is the commutator of its predecessor with the subgroup, gets inside the given subgroup after finitely many steps. The number of steps is takes is termed the subnormal depth.

The equivalence of the definitions thus must also show that the notions of subnormal depth in each case are the same.

A k-subnormal subgroup is a subnormal subgroup with subnormal depth at most k.

Definition with symbols

A subgroup H is termed subnormal in a group G if either of the following equivalent conditions holds:

  1. There exists an ascending chain H=H0≤H1≤…≤Hn=G such that each Hi is normal in Hi+1. The smallest possible n for which such a chain exists is termed the subnormal depth of H.
  2. Consider the descending chain Gi defined as follows: G0=G and Gi+1 is the normal closure of H in Gi. Then, there exists an n for which Gn=H. The smallest such n is termed the subnormal depth of H.
  3. Consider the sequence Ki of subgroups of G defined as follows: K0=G, and Ki+1=[H,Ki] (the commutator), This sequence of subgroups eventually enters inside H. The number of steps taken is termed the subnormal depth of H.

A k-subnormal subgroup is a subnormal subgroup with subnormal depth at most k.

Equivalence of definitions

For full proof, refer: Equivalence of definitions of subnormal subgroup

Examples

VIEW: subgroups of groups satisfying this property | subgroups of groups dissatisfying this property
VIEW: Related subgroup property satisfactions | Related subgroup property dissatisfactions

Formalisms

In terms of the subordination operator

This property is obtained by applying the subordination operator to the property: normal subgroup
View other properties obtained by applying the subordination operator

The property of being a subnormal subgroup is obtained by applying the subordination operator to the group property of normality.

Facts

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 (by definition) If H≤K≤G, with H subnormal in K and K subnormal in G, then H is subnormal in G.
trim subgroup property Yes (by definition) The trivial subgroup and the whole group are subnormal in the group.
finite-intersection-closed subgroup property Yes Subnormality is finite-intersection-closed If H1,H2 are subnormal subgroups of G, then H1∩H2 is also subnormal in G.
finite-relative-intersection-closed subgroup property Yes Subnormality is finite-relative-intersection-closed In other words, if H,K≤G are subgroups such that H is h-subnormal in G and K is k-subnormal in some subgroup L of G containing both H and K, then H∩K is (h+k)-subnormal in G.
intermediate subgroup condition Yes Subnormality satisfies intermediate subgroup condition If H≤K≤G with H subnormal in G, then H is subnormal in K.
finite-join-closed subgroup property No Subnormality is not finite-join-closed It is possible to have subnormal subgroups H,K of a group G such that ⟨H,K⟩ is not subnormal in G.
normalizing join-closed subgroup property Yes Subnormality is normalizing join-closed If H,K≤G are both subnormal subgroups and K≤NG(H), then HK is also a subnormal subgroup.
permuting join-closed subgroup property Yes Subnormality is permuting join-closed If H,K≤G are both subnormal subgroups and HK=KH, then HK is also a subnormal subgroup.
transfer condition Yes Subnormality satisfies transfer condition If H,K≤G with H subnormal, then H∩K is subnormal in K.
centralizer-closed subgroup property No Subnormality is not centralizer-closed It is possible to have H≤G subnormal such that CG(H) is not subnormal in G.
commutator-closed subgroup property No Subnormality is not commutator-closed It is possible to have subnormal subgroups H,K≤G such that the commutator [H,K] is not subnormal in G.
finite-upper join-closed subgroup property No Subnormality is not finite-upper join-closed It is possible to have H≤G with intermediate subgroups K1,K2 such that H is subnormal in K1 and in K2 but not in ⟨K1,K2⟩.
conditionally lattice-determined subgroup property No No subgroup property between normal Sylow and subnormal or between Sylow retract and retract is conditionally lattice-determined It is possible to have a group G, an automorphism φ of the lattice of subgroups, and a subnormal subgroup H of G such that φ(H) is not subnormal.

Relation with other properties

Stronger properties

Property Meaning Proof of implication Proof of strictness (reverse implication failure) Intermediate notions
Normal subgroup subnormal of depth 1 (obvious) subnormal not implies normal (also see normality is not transitive, there exist subgroups of arbitrarily large subnormal depth) |FULL LIST, MORE INFO
Finitarily hypernormalized subgroup taking the normalizer finitely many times yields the whole group follows from abnormal normalizer and 2-subnormal not implies normal |FULL LIST, MORE INFO
2-subnormal subgroup normal subgroup of normal subgroup (obvious) there exist subgroups of arbitrarily large subnormal depth |FULL LIST, MORE INFO
3-subnormal subgroup normal subgroup of normal subgroup of normal subgroup (obvious) there exist subgroups of arbitrarily large subnormal depth |FULL LIST, MORE INFO
4-subnormal subgroup normal of normal of normal of normal (obvious) there exist subgroups of arbitrarily large subnormal depth |FULL LIST, MORE INFO
Join-transitively subnormal subgroup join with any subnormal subgroup is subnormal because the trivial subgroup is subnormal subnormality is not finite-join-closed |FULL LIST, MORE INFO
Left-transitively fixed-depth subnormal subgroup subnormal not implies left-transitively fixed-depth subnormal |FULL LIST, MORE INFO
Right-transitively fixed-depth subnormal subgroup subnormal not implies right-transitively fixed-depth subnormal |FULL LIST, MORE INFO
Linear-bound join-transitively subnormal subgroup join with a k-subnormal subgroup is subnormal of depth bounded by a linear function of k
Polynomial-bound join-transitively subnormal subgroup join with a k-subnormal subgroup is subnormal of depth bounded by a polynomial function of k
Subnormal subgroup of finite group
Permutable subgroup of finite group permutable implies subnormal in finite
Conjugate-permutable subgroup of finite group conjugate-permutable implies subnormal in finite

Weaker properties

Property Meaning Proof of implication Proof of strictness (reverse implication failure) Intermediate notions
Ascendant subgroup ascending (possibly transfinite) chain from subgroup to whole group, each normal in successor subnormal implies ascendant ascendant not implies subnormal |FULL LIST, MORE INFO
Descendant subgroup subnormal implies descendant descendant not implies subnormal |FULL LIST, MORE INFO
Serial subgroup (via ascendant, descendant) (via ascendant, descendant) |FULL LIST, MORE INFO
Subpronormal subgroup |FULL LIST, MORE INFO
Locally subnormal subgroup subnormal in its join with any finitely generated subgroup |FULL LIST, MORE INFO
Almost subnormal subgroup ascending chain from subgroup to whole group with each member either normal or of finite index in successor |FULL LIST, MORE INFO
Join of finitely many subnormal subgroups join of finitely many subnormal subgroups
Join of subnormal subgroups join of subnormal subgroups
Upper join of subnormal subgroups the whole group is a join of intermediate subgroups in each of which the given subgroup is subnormal.

Conjunction with other properties

In fact, there are a large number of subgroup properties whose conjunction with subnormality gives normality. Further information: subnormal-to-normal and normal-to-characteristic

Conjunction with group properties:

The property of being subnormal in particular kinds of groups is also of interest:

Related group properties

Effect of property operators

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)

Operator Meaning Result of application Proof
left transiter Whenever the whole group is subnormal in some bigger group, so is the subgroup. subnormal subgroup subnormality is a t.i. subgroup property, so equals its own left transiter.
right transiter Any subnormal subgroup of the subgroup is subnormal in the whole group. subnormal subgroup subnormality is a t.i. subgroup property, so equals its own right transiter.
subordination operator There is a finite series of subgroups from the subgroup to the whole group with each subnormal in the next. subnormal subgroup subnormality is a t.i. subgroup property, so equals its own subordination.
join-transiter The join with any subnormal subgroup is subnormal. join-transitively subnormal subgroup (by definition)
finite-join-closure operator The join in the whole group of finitely many subnormal subgroups. join of finitely many subnormal subgroups (by definition)
join-closure operator The join in the whole group of subnormal subgroups. join of subnormal subgroups (by definition)
intersection-closure operator The intersection in the whole group of subnormal subgroups intersection of subnormal subgroups (by definition)

More on these property operators: [SHOW MORE]

Testing

The testing problem

Further information: subnormality testing problem

The property of being subnormal can be tested for permutation groups. In fact, it can be tested if we have an algorithm for normal closure-finding.

GAP command

This subgroup property can be tested using built-in functionality of Groups, Algorithms, Programming (GAP).
The GAP command for testing this subgroup property is:IsSubnormal
View subgroup properties testable with built-in GAP command|View subgroup properties for which all subgroups can be listed with built-in GAP commands | View subgroup properties codable in GAP
Learn more about using GAP

The GAP syntax for determining whether a subgroup is subnormal is: 

IsSubnormal (Group, Subgroup);

The GAP syntax for finding a subnormal series for a given subgroup is: 

SubnormalSeries(Group, Subgroup);

The subnormal depth can be determined by the command: 

Length(SubnormalSeries(Group, Subgroup)) - 1;

The -1 is necessary because the length of a series, in GAP, is the number of terms rather than the number of ascent/descent symbols.

Study of the notion

Mathematical subject classification

Under the Mathematical subject classification, the study of this notion comes under the class: 20E15

Under the Mathematical subject classification, the study of a slight variant of this notion comes under the class: 20D35

The class 20D35 studies subnormal subgroups for finite groups. In the infinite case, 20E15 is more appropriate.

References

Textbook references

  • A Course in the Theory of Groups by Derek J. S. Robinson, ISBN 0387944613More info, Page 63 (definition introduced in the context of a more general definition)
  • Groups and representations by Jonathan Lazare Alperin and Rowen B. Bell, ISBN 0387945261More info, Page 92-93 (introduces notion of subnormal series, does not explicitly talk of subnormal subgroup)
  • Finite Group Theory (Cambridge Studies in Advanced Mathematics) by Michael Aschbacher, ISBN 0521786754More info, Page 23 (formal definition)
  • Finite Groups by Daniel Gorenstein, ISBN 0821843427More info, Page 13, Chapter 1, Exercise 5 (definition introduced in exercise)

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Definition links

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