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Math Help - free groups, finitely generated groups

  1. #1
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    free groups, finitely generated groups

    I have three problems I wasn't able to solve by myself.

    1) Is ( \mathbb{R}^{*}, .) finitely generated group? Is it free?

    2) Is \mathbb{Z}_3 \oplus \mathbb{Z} free Abelian group?

    3) Is \mathbb{Z}_6 free group?

    I know that in order to check whether a group is finitely generated, I have to find a finite set that generates it, or prove it doesn't exist, but I can't do either.

    Also, to check whether something is a free group, I would have to find its basis.
    For example, is \{0, 1, 2, 3, 4, 5 \} the basis of \mathbb{Z}_6?

    I'm really lost, please help.
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  2. #2
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    Quote Originally Posted by georgel View Post
    I have three problems I wasn't able to solve by myself.

    1) Is ( \mathbb{R}^{*}, .) finitely generated group? Is it free?
    it's not free because it's not torsion-free. for example -1 has finite order. (recall that every free abelian group is torsion-free)

    it's not finitely generated because \mathbb{Q}^*, which is a subgroup of \mathbb{R}^*, is not finitely generated. (recall that every subgroup of a finitely generated abelian group is finitely generated)


    2) Is \mathbb{Z}_3 \oplus \mathbb{Z} free Abelian group?
    it's not free because it's not torsion-free. for example (1,0) has finite order.

    it is finitely generated because it's generated by (1,0) and (0,1).


    3) Is \mathbb{Z}_6 free group?
    no it's not! a non-trivial finite group is never free!
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  3. #3
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    Thank you so much!!!

    Quote Originally Posted by NonCommAlg View Post
    no it's not! a non-trivial finite group is never free!
    Could you possibly explain why this is so?
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  4. #4
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    Quote Originally Posted by georgel View Post
    1) Is ( \mathbb{R}^{*}, .) finitely generated group? Is it free?

    Further to NCA's answer, I would like to point out that no uncountably infinite group is finitely generated. This is because every product is, by definition, of finite length and so all products can be "counted" using the natural numbers.

     \mathbb{R} is uncountably infinite and so is not finitely generated.
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  5. #5
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    Quote Originally Posted by georgel View Post
    Thank you so much!!!

    Could you possibly explain why this is so?
    in a free group, the elements of a basis they all have infinite order but a finite group has no element of infinite order!

    what Swlabr said is true and it works for your problem, but it doesn't prove that \mathbb{Q}^* is not finitely generated (which is a good exercise! ).
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  6. #6
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    Quote Originally Posted by NonCommAlg View Post
    in a free group, the elements of a basis they all have infinite order but a finite group has no element of infinite order!

    what Swlabr said is true and it works for your problem, but it doesn't prove that \mathbb{Q}^* is not finitely generated (which is a good exercise! ).
    I'm terribly ashamed I din't think of that.

    Thanks to both you and Swlabr!

    And I already proved that \mathbb{Q}^* is not finitely generated!
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