Proving Finite Groups with No Non-Trivial Subgroups and Prime Order o(G)

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Discussion Overview

The discussion centers on proving that a group G with no non-trivial subgroups must be finite and have a prime order. Participants explore the implications of the absence of non-trivial subgroups and the relationship between the order of G and its subgroup structure.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant asserts that if G is infinite, it can be shown that there are non-trivial subgroups, suggesting that finiteness is a necessary condition.
  • Another participant proposes that if G has no non-trivial subgroups, then for any element x not equal to the identity e, the subgroup generated by x, denoted , must equal G.
  • A subsequent reply reiterates that since G has no non-trivial subgroups and is a subgroup of G, G must equal , implying G is cyclic.
  • Another participant claims that if G is a finite cyclic group with order m, then for every divisor d of m, there exists a subgroup H of order d, which leads to the conclusion that if only trivial subgroups exist, m must be prime.
  • One participant expresses agreement with the previous point, indicating a positive reception of the argument presented.

Areas of Agreement / Disagreement

Participants generally agree on the implications of the absence of non-trivial subgroups leading to the conclusion that G must be finite and of prime order. However, the discussion includes varying approaches to proving these claims, and no consensus is reached on the completeness of the arguments presented.

Contextual Notes

The discussion does not resolve the mathematical steps regarding the implications of cyclic groups and their subgroups, nor does it clarify the assumptions about the nature of G beyond the stated conditions.

QuArK21343
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Prove that if a group G has no non-trivial subgroups, G is finite and o(G) is a prime number, where o(G) is the order of the group G.

If G is infinite, you can show that there are non trivial subgroups. What remains to prove is that if o(G) is not prime, than there is at least one subgroup H, with o(H) equal to one of the prime divisor of o(G). Any idea?
 
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Suppose that G has no nontrivial subgroups. Take an arbitrary element x with [itex]x\neq e[/itex]. Then what can you say about <x> (the group generated by x)??
 
Since G has no nontrivial subgroup and <x> is a subgroup of G, but <x> is not e, G=<x>.
 
QuArK21343 said:
Since G has no nontrivial subgroup and <x> is a subgroup of G, but <x> is not e, G=<x>.

So you must only prove now that all cyclic subgroups whose order is not prime have a nontrivial subgroup...
 
I prove that if G=<g> is a finite cyclic group and o(G)=m, then for every d such that d|m there is a subgroup H such that o(H)=d. It is simply this: consider the subgroup <g^(m/d)>. Since o(G)=o(g)=m, we have that o(H)=o(g^(m/d))=m/(m/d)=d. So, H is a non trivial subgroup. So, if the only possible subgroups are the trivial ones, m must be prime.
 
Seems ok! :smile:
 

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