# T-group

This article defines a group property: a property that can be evaluated to true/false for any given group, invariant under isomorphism
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## History

### Origin

Ernest Best and Olga Taussky defined these groups in their paper A class of Groups, published in 1942. They named them t-groups.

Wolfgang Gaschütz described the soluble T-groups in 1957 as the groups G in which the nilpotent residual is an abelian Hall subgroup L of odd order such that G/L is Dedekind and G normalizes every subgroup of L.

The notion of T-group was discussed by Derek J.S. Robinson in his paper A Note on Finite Groups in which normality is transitive published in 1968.

## Definition

A group is termed a T-group if it satisfies the following equivalent conditions:

No. Shorthand A group is termed a T-group if ... A group $G$ is termed a T-group if ...
1 subnormal equals normal any subnormal subgroup of the group is normal in the group. for any subgroup $H \le G$ such that there exists an ascending chain $H = H_0 \le H_1 \le H_2 \le \dots \le H_n = G$ where each $H_i$ is normal in $H_{i+1}$, we have that $H$ is normal in $G$.
2 2-subnormal equals normal any 2-subnormal subgroup (i.e. any normal subgroup of a normal subgroup) is normal in the group. for any subgroups $H \le K \le G$ such that $H$ is normal in $K$ and $K$ is normal in $G$, we have that $H$ is normal in $G$.
3 normal equals transitively normal any normal subgroup is transitively normal for any normal subgroup $K$ of $G$, it is true that any normal subgroup of $K$ is normal in $G$.
This definition is presented using a tabular format. |View all pages with definitions in tabular format

## Examples

### Groups satisfying the property

Here are some basic/important groups satisfying the property:

GAP ID
Cyclic group:Z22 (1)
Cyclic group:Z33 (1)
Cyclic group:Z44 (1)

Here are some relatively less basic/important groups satisfying the property:

GAP ID
Alternating group:A560 (5)
Alternating group:A6360 (118)
Direct product of Z4 and Z28 (2)
Projective special linear group:PSL(3,2)168 (42)
Quaternion group8 (4)
Special linear group:SL(2,5)120 (5)
Symmetric group:S5120 (34)

Here are some even more complicated/less basic groups satisfying the property:

GAP ID
Alternating group:A7
Special linear group:SL(2,7)336 (114)
Special linear group:SL(2,9)720 (409)

### Groups dissatisfying the property

Here are some basic/important groups that do not satisfy the property:

Here are some relatively less basic/important groups that do not satisfy the property:

GAP ID
Alternating group:A412 (3)
Dihedral group:D88 (3)
Special linear group:SL(2,3)24 (3)
Symmetric group:S424 (12)

Here are some even more complicated/less basic groups that do not satisfy the property:

GAP ID
Binary octahedral group48 (28)
Dihedral group:D1616 (7)
Direct product of D8 and Z216 (11)
General linear group:GL(2,3)48 (29)
Mathieu group:M972 (41)
Semidihedral group:SD1616 (8)

## Metaproperties

Metaproperty name Satisfied? Proof Statement with symbols
subgroup-closed group property No T-group property is not subgroup-closed It is possible to have a T-group $G$ and a subgroup $H$ of $G$ such that $H$ is not a T-group. (A T-group for which every subgroup is a T-group is termed a T*-group.
quotient-closed group property Yes T-group property is quotient-closed Suppose $G$ is T-group and $H$ is a normal subgroup of $G$. Then, the quotient group $G/H$ is also a T-group.
normal subgroup-closed group property Yes T-group property is normal subgroup-closed. Suppose $G$ is T-group and $H$ is a normal subgroup of $G$. Then, $H$ is also a T-group.

## Formalisms

### In terms of the subgroup property collapse operator

This group property can be defined in terms of the collapse of two subgroup properties. In other words, a group satisfies this group property if and only if every subgroup of it satisfying the first property (normal subgroup) satisfies the second property (transitively normal subgroup), and vice versa.
View other group properties obtained in this way

The property of being a T-group can be viewed as any of these subgroup property collapses:

### In terms of the transitivity-forcing operator

This property is obtained by applying the transitivity-forcing operator to the property: normal subgroup
View other properties obtained by applying the transitivity-forcing operator

## Relation with other properties

### Stronger properties

Property Meaning Proof of implication Proof of strictness (reverse implication failure) Intermediate notions
Dedekind group every subgroup is normal (direct from definition) simple non-abelian groups provide counterexamples Group in which every subgroup is pronormal, T*-group|FULL LIST, MORE INFO
abelian group any two elements commute (via Dedekind) (via Dedekind) Group in which every normal subgroup is a central factor, Group in which every subgroup is pronormal, T*-group|FULL LIST, MORE INFO
T*-group every subgroup is a T-group by definition T-group property is not subgroup-closed |FULL LIST, MORE INFO
simple group no proper nontrivial group by definition any abelian non-simple group gives a counterexample Group in which every normal subgroup is a direct factor|FULL LIST, MORE INFO

## Effect of property operators

### Subgroup-closure

The property of being a group such that every subgroup of it is a T-group, is termed the property of being a T*-group.

## Testing

### GAP code

One can write code to test this group property in GAP (Groups, Algorithms and Programming), though there is no direct command for it.
View the GAP code for testing this group property at: IsTGroup
View other GAP-codable group properties | View group properties with in-built commands

There is no built-in GAP command to check whether a group is a T-group, but a short snippet of code, available at GAP:IsTGroup, can achieve this.