Are All Indecomposable Groups Cyclic?

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Discussion Overview

The discussion revolves around the properties of indecomposable groups, specifically whether all indecomposable groups are cyclic and whether all cyclic groups are indecomposable. The scope includes theoretical aspects of group theory and properties of finite abelian groups.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant defines an indecomposable group and provides an example of groups of prime order being cyclic.
  • Another participant suggests looking at "simple" groups as a relevant category in this context.
  • A different participant notes that within finite abelian groups, all indecomposable groups are cyclic, but not all cyclic groups are indecomposable, highlighting that finite abelian groups can decompose into products of indecomposable cyclic factors.
  • One participant expresses appreciation for notes shared by another, indicating interest in the extension to linear mappings.
  • Another participant reflects on their experience with linear algebra and its analogies to finite abelian groups.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between indecomposable and cyclic groups, with some asserting that not all cyclic groups are indecomposable while others focus on specific cases like finite abelian groups. The discussion remains unresolved regarding the general question of whether all indecomposable groups are cyclic.

Contextual Notes

The discussion includes limitations related to the definitions of indecomposable and cyclic groups, as well as the specific context of finite abelian groups, which may not apply universally.

metapuff
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A group is said to be indecomposable if it cannot be written as a product of smaller groups. An example of this is any group of prime order p, which is isomorphic to the group of integers modulo p (with addition as the group operation). Since the integers modulo p is a cyclic group (generated by 1), we have that any indecomposable group of prime order is cyclic. I have two questions:

Are ALL indecomposable groups cyclic? (N.B. not just those of prime order)

Are all cyclic groups indecomposable?

Thanks!
 
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Look at a list of "simple" groups.
 
If you restrict to finite abelian groups, then all indecomposable such groups are cyclic, but not all cyclic groups are indecomposable. All finite abelian groups decompose into a product of not just cyclic factors, but of indecomposable cyclic factors.

see my notes, especially pages 42-43:

http://www.math.uga.edu/%7Eroy/4050sum08.pdf
 
Ah, thanks @mathwonk! Your notes turned out to be just the reference I was looking for. I thought the extension to linear mappings was pretty interesting.
 
you are welcome. i had fun thinking that stuff through for a summer course in linear algebra, especially the analogies with finite abelian groups.
 

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