What Makes Freely Indecomposable Groups Essential in Mathematics?

  • Context: Graduate 
  • Thread starter Thread starter Oxymoron
  • Start date Start date
Click For Summary

Discussion Overview

The discussion revolves around the concept of freely indecomposable groups in group theory, exploring their definitions, properties, and significance. Participants examine the implications of being freely indecomposable, compare it to other mathematical constructs, and question various aspects of the definitions involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants seek clarification on the definition of freely indecomposable groups, questioning the use of equals signs versus isomorphisms in the context of free products.
  • One participant suggests that freely indecomposable groups cannot be expressed as direct products of smaller free groups, emphasizing their role as building blocks in group theory.
  • A comparison is drawn between freely indecomposable groups and prime numbers, with one participant noting the analogy in terms of being fundamental components.
  • There is uncertainty about the definitions of finite and simple groups, with some participants expressing gut feelings about their indecomposability and later revising their thoughts based on discussion.
  • Another participant asserts that finite groups cannot be written as free products of non-trivial groups, prompting questions about the reasoning behind this assertion.
  • Concerns are raised regarding the understanding of free products, with one participant challenging another's reasoning about the normality of groups within free products.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and properties of finite and simple groups, leading to some confusion and revision of initial thoughts. There is no consensus on the implications of these definitions or their relationship to freely indecomposable groups.

Contextual Notes

Participants acknowledge limitations in their understanding of the definitions and properties discussed, particularly regarding the nature of free products and the implications for finite and simple groups. Some statements reflect evolving thoughts rather than settled conclusions.

Who May Find This Useful

This discussion may be of interest to those studying group theory, particularly in understanding the properties and significance of freely indecomposable groups and their relationships to other mathematical constructs.

Oxymoron
Messages
868
Reaction score
0
Can anyone explain why a freely indecomposable group is important or handy?

I can't seem to understand the definition. It says that a group G is free indecomposable if

G = A \star B \Rightarrow G = A \vee G = B

So is it really saying that a free product A * B is freely indecomposable if

A \star B \Rightarrow A \star B = A \vee A \star B = B?
 
Physics news on Phys.org
I've never seen it before, but I think you're wrong to use equals signs, and they should be isomorphisms. Anyway, if G is free indecomposable it cannot be written as the direct product of strictly smaller free groups. And it is always important to know what the indecomposable objects in your theory are since they are by definition the building blocks of all objects in your theory.
 
Posted by Matt Grime:

...if G is free indecomposable it cannot be written as the direct product of strictly smaller free groups. And it is always important to know what the indecomposable objects in your theory are since they are by definition the building blocks of all objects in your theory.

This sounds like prime numbers! "If an integer is prime it cannot be written as a product of strictly smaller integers." Is there meant to be an analogy here? Besides, prime numbers are thought of as building blocks for the integers.
 
What is a finite group? Is it one which is simply generated by finitely many generators? I would say that a finite group is indecomposable. I have no clue as to why at this stage though. This is just a gut feeling.

What is a simple group? Is it simply a non-trivial group with no non-trivial normal subgroups? I would guess that a simple group is not indecomposable.

EDIT: Actually I take that back. I think that if G \equiv A \star B is simple then it is freely indecomposable. Because if G is simple then it has no non-trivial normal subgroups.
 
Last edited:
A finite group is one with finitely many elements (*not* generators).

Anyway, it is clearly impossible to write a finite group as the free product of two non-trivial groups, so a finite group is, I suppose, free indecomposable.

The integers are also freely indecomosable with that definition.

Since there are finite simple groups your two gut feelings are contrdictory.

Z^2 is also free indecomposable, I believe. F_2 is not, nor is any free group with more than 2 generators.It is probably easier to describe what a 'free decomposition' of a group is.

G is freely decomposable if we can pick a partition of a set of generators of G into I and J, with G= <I>*<J> (<I> is the group generated by I).

I.e. we can split generators for G into two subsets which have no relations between them.
 
Last edited:
Posted by Matt Grime:

...it is clearly impossible to write a finite group as the free product of two non-trivial group...

Why is this clear? Why will I be unable to find two non-trivial groups whose free product will be finite? Is it because of how the free product is constructed?

Posted by Matt Grime:

Since there are finite simple groups your two gut feelings are contrdictory.

Did you post this after my edit? My gut now thinks that both simple and finite groups are freely indecomposable. And thanks to you I almost understand why the finite groups are freely indecomposable.
 
Just look at the definition of a free product. If G=A*B and A and B are non-trival, then there are nonidentity a and b in A and B. Now what can you say about ab,abab,ababab,ababababab, etc?
 
I don't follow your reasoning about simple groups. If G=A*B, then that does not say that A is normal in G. Indeed, it is clearly not normal *by the definition of free product*. I am almost forced to the conclusion that you do not know what a free product really is.
 

Similar threads

  • · Replies 14 ·
Replies
14
Views
4K
  • · Replies 1 ·
Replies
1
Views
992
  • · Replies 3 ·
Replies
3
Views
1K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 4 ·
Replies
4
Views
5K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 7 ·
Replies
7
Views
3K
  • · Replies 19 ·
Replies
19
Views
4K
  • · Replies 3 ·
Replies
3
Views
2K