Proving G is Cyclic if No Subgroups Other than G and {e} Exist

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In summary, the conversation discusses how to prove that a group G is cyclic if it has no subgroup other than G and {e}. The solution involves constructing <a> - a cyclic subgroup generated by an element a. It is noted that <a> must be equal to G, and therefore a generates G, fulfilling the definition of a cyclic group. The conversation concludes by correcting a mistake and emphasizing that <a> is not equal to {e} but rather G.
  • #1
vedicguru
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Homework Statement



Prove that if a group G has no subgroup other than G and {e}, then G is cyclic...

Homework Equations





The Attempt at a Solution



we could say that, let a E G - {e} then we construct <a>...
 
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  • #2
Sounds good. Why are you having doubts?
 
  • #3
how do I construct <a> ??
 
  • #4
Well, you were the one who used that notation- what is the definition of "<a>"?
 
  • #5
Huh? Well by definition, <a>={..., a^-1,e,a,a²,a³,...}. Since <a> is a subgroup by construction and since it is not equal to {e}, it is equal to G.
 
  • #6
oh yeah sure! I meant how would say, after constructing it! I forgot it is equal to {e} it is equal to G...this is what I get!
We could say that, let a E G - {e} then we construct <a> =
a, a^2, a^3... a^n=1 <- cyclic subgroup generated by element a.
Since <a> is subgroup, it must coincide with G (because G is the only subgroup), but then a generates G: <a> = G, which is definition of G being cyclic...
are we done after saying this statement or should we have to mention something else?
thanks for offering help!
 
  • #7
Well what you wrote for the definition of <a> is incorrect, and don't forget to say "since <a> is not {e}, it is G".
 
  • #8
ohh yeah I am sorry about that...I was in hurry or something to write it down! but yeah I believe everything else is fine and I would not forget to mention that <a> is not equal to {e}..it is G!
Thanks!
 

Related to Proving G is Cyclic if No Subgroups Other than G and {e} Exist

1. How do you prove that a group is cyclic?

To prove that a group G is cyclic, we must show that there exists an element g in G such that every other element in G can be written as a power of g. This can be done by finding a generator for the group, or by showing that there are no subgroups other than G and the identity element in G.

2. What does it mean for a group to be cyclic?

A cyclic group is a group where every element can be generated by a single element, known as a generator. This means that the group can be written as a sequence of powers of the generator, and that there are no subgroups other than the group itself and the identity element.

3. Can a group be cyclic if it has other subgroups?

No, if a group has any subgroups other than the group itself and the identity element, it cannot be cyclic. This is because a cyclic group must be generated by a single element, and any additional subgroups would contradict this definition.

4. How does the absence of subgroups prove that a group is cyclic?

If a group G has no subgroups other than G and the identity element, then every element in G must be generated by a single element. This means that G is cyclic, as every element can be written as a power of this generator.

5. Can a non-cyclic group have no subgroups other than itself and the identity element?

No, a non-cyclic group must have at least one subgroup other than itself and the identity element. This is because a non-cyclic group cannot be generated by a single element, so there must be at least one other subgroup in order for every element to be generated.

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