# Epicenter

This article defines a subgroup-defining function, viz., a rule that takes a group and outputs a unique subgroup
View a complete list of subgroup-defining functions OR View a complete list of quotient-defining functions

## Definition

The epicenter (also called epicentre, precise center, or precise centre) of a group $G$, sometimes denoted $Z^*(G)$, is defined in the following equivalent ways:

1. It is the intersection, over all central extensions $E$ with quotient group $G$, of the images in the group of the center of the central extension group. Explicitly, it is the intersection $\bigcap_{(E,\varphi)} \varphi(Z(E))$ where $(E,\varphi)$ describes a central extension with quotient $G$, $E$ being the extension group and $\varphi:E \to G$ the quotient map.
2. It is the unique largest normal subgroup of the group for which the quotient group is a capable group.
3. It is the subset of $G$ comprising those elements $g \in G$ such that, for all $h \in G$, $g \wedge h$ is the identity element of the exterior square of $G$.
4. Suppose $E$ is a Schur covering group of $G$ with covering map $\varphi:E \to G$. The epicenter of $G$ is defined as $\varphi(E)$.

### Equivalence of definitions

Further information: equivalence of definitions of epicenter

## Particular cases

Group property Computing the epicenter for such a group
finite group the epicenter can also be defined as the intersection of the kernels of all projective representations over the complex numbers, where the kernel of a projective representation is understood to mean the kernel of the corresponding homomorphism to the projective general linear group.
finite abelian group characterization of epicenter of finite abelian group -- the epicenter can be described and computed explicitly.

## Related group properties

Epicenter Corresponding group property What it means for a group $G$
trivial subgroup capable group $G$ is isomorphic to the inner automorphism group of some group, i.e., $G \cong K/Z(K)$ for some group $K$.
whole group epabelian group If $G$ is isomorphic to the quotient of some group $K$ by a central subgroup $H$ of $K$, then $K$ is abelian.
center unicentral group It means that the quotient by any proper subgroup of the center is not capable.

## Subgroup properties

### Properties satisfied

Property Meaning Proof of satisfaction
normal subgroup invariant under all inner automorphisms follows from being characteristic, also follows from being central
characteristic subgroup invariant under all automorphisms follows from subgroup-defining function value is characteristic
central subgroup contained inside the center

### Properties not satisfied

The properties below are not always satisfied by the epicenter in a group. There may be groups where the epicenter satisfies the property.

Property Meaning Proof of satisfaction
fully invariant subgroup invariant under all endomorphisms epicenter not is fully invariant

## Relation with other subgroup-defining functions

• Epicentral series. Note that this is not the quotient-iterated series for the epicenter.

## Computation

### GAP command

The command for computing this subgroup-defining function in Groups, Algorithms and Programming (GAP) is:Epicenter
View other GAP-computable subgroup-defining functions

This command requires the Hap package, so if the package is not loaded, you need to load it as follows: