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Math Help - subgroup/group theory

  1. #1
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    Feb 2009
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    subgroup/group theory

    let H and K be subgroups of G. Prove that H U K is a subgroup iff either H is a subset of K or K is a subset of H?

    just having trouble with this problem. appreciate a few pointers.

    Thank you!
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  2. #2
    Member
    Joined
    Feb 2009
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    Chennai
    Posts
    148
    hi--

    i give u a hint

    there will be 2 parts to this question.

    Assume: H \cup K is a subgroup. Think what happens when H is not contained in K or vice versa.

    Converse: this is very easy. Take x,y \in H or K. Since H,K are subgroups the statement follows
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