Centerless Groups: Examples & Constraints

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Discussion Overview

The discussion revolves around the concept of centerless groups, particularly in the context of matrix groups and their centers. Participants explore examples of centerless groups, the properties that define them, and the constraints that must be relaxed to achieve such groups. The conversation includes both theoretical and practical considerations related to group theory.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that matrix groups, defined by multiplication, inherently possess a center that includes both the identity I and -I, leading to the conclusion that they cannot be centerless.
  • Another participant points out that a centerless group is defined as having a trivial center, specifically Z=I, which excludes -I.
  • A participant provides the example of the symmetry group D_3, which has a trivial center, and discusses its representation through specific matrix forms.
  • There is a query about the center of SU(2), which includes both I and -I, and how this relates to the commutation with other elements in the group.
  • One participant argues that the presence of -I in a matrix group does not affect the center of other matrix groups, emphasizing that the center is defined by the elements contained within the group.
  • Another participant clarifies that if a group does not include -I, then the commutation with -I is irrelevant to the definition of the center.

Areas of Agreement / Disagreement

Participants express differing views on the implications of matrix groups having a center that includes -I and the conditions under which a group can be centerless. There is no consensus on the examples of centerless groups or the constraints that must be relaxed to achieve them.

Contextual Notes

Participants discuss the definitions and properties of group centers, but there are unresolved assumptions regarding the nature of matrix groups and their elements. The implications of including or excluding specific elements like -I in defining the center remain contested.

gentsagree
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I imagine a matrix group, with multiplication as the composition rule, to always possesses the quality of having centre (I,-I), as I can't see when both elements wouldn't commute with all others. On the other hand, though, a centerless group is defined as having trivial centre, i.e. Z=I (which means, Z doesn't include -I).

I imagine non-matrix groups could show this property, but I can't think of any.

Could somebody give a couple of examples of centreless groups, and what "constraints" must be relaxed (from my matrix group example above) in order to achieve them?
 
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gentsagree said:
I imagine a matrix group, with multiplication as the composition rule, to always possesses the quality of having centre (I,-I),

A matrix group must contain the identity [itex]I[/itex], but need not contain [itex]-I[/itex].

as I can't see when both elements wouldn't commute with all others. On the other hand, though, a centerless group is defined as having trivial centre, i.e. Z=I (which means, Z doesn't include -I).

I imagine non-matrix groups could show this property, but I can't think of any.

Could somebody give a couple of examples of centreless groups, and what "constraints" must be relaxed (from my matrix group example above) in order to achieve them?

The group [itex]D_3[/itex], which is the symmetry group of an equilateral triangle, has trivial center. The group has a 2-dimensional representation generated by a reflection in the x-axis
[tex] M_m = \begin{pmatrix} -1 & 0 \\ 0 & 1 \end{pmatrix}[/tex]
and a rotation through [itex]2\pi/3[/itex] about the origin,
[tex] M_\rho = \begin{pmatrix} \cos(2\pi/3) & -\sin(2\pi/3) \\ \sin(2\pi/3) & \cos(2\pi/3) \end{pmatrix}[/tex]
 
Thanks, I see how it would work for D3. However, I was referring to the center of SU(2), which is (I,-I).

If the composition rule is multiplication, how is it possible to find an element of the group which doesn't commute with -I?
 
gentsagree said:
Thanks, I see how it would work for D3. However, I was referring to the center of SU(2), which is (I,-I).

SU(2) is a matrix group. There are other groups of 2x2 matrices, and the fact that the center of SU(2) is {I, -I} has no bearing on the center of any other matrix group.

If the composition rule is multiplication, how is it possible to find an element of the group which doesn't commute with -I?

It isn't, but that doesn't matter if the group in question doesn't contain -I in the first place!

The center of a group [itex]G[/itex] consists exactly of those [itex]g \in G[/itex] such that for all [itex]h \in G[/itex], [itex]gh = hg[/itex].

Thus, if [itex]G[/itex] is a matrix group and [itex]-I \notin G[/itex], we don't care that [itex](-I)M = M(-I)[/itex] for all [itex]M \in G[/itex]; [itex]-I[/itex] fails to be in the center of [itex]G[/itex] by virtue of not being in [itex]G[/itex] in the first place.
 

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