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Math Help - Cylic

  1. #1
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    Cylic

    1. Suppose G = Z12 x Z12 and H =<(2,2)> is the subgroup generated by (2,2). Find the order of the element (5, 8) + H in G/H and prove it. Is G/H cyclic? Explain.

    2. Find a direct product of cyclic groups of prime power order that is isomorphic to (Z9 x Z9)/<(3,3)>, prove your answer.

    Could anyone help me? please!!
    Last edited by heha010101; November 30th 2011 at 09:42 AM.
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  2. #2
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    Re: Cylic

    1. this is just straight-forward calculation:

    H = {(2,2), (4,4), (6,6), (8,8), (10,10), (0,0)}. this has 6 elements, so so does (5,8) + H:

    {(7,10), (9,0), (11,2), (1,4), (3,6), (5,8)}.

    calculating powers of (5,8) + H, we get:

    2[(5,8) + H] = (10,4) + H
    3[(5,8) + H] = (3,0) + H
    4[(5,8) + H] = (8,8) + H = H, so (5,8) + H is of order 4.

    for G/H to be cyclic, it would need to have an element of order 144/6 = 24.

    but G/H has no element of order > 12, since 12[(a,b) + H] = (12a, 12b) + H = (0,0) + H = H.

    2. what have you tried so far?
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  3. #3
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    Re: Cylic

    Thank you so much for the first one.

    And for the second one
    I'm kinda clueless

    I know <(3, 3)> is the cylic subgroup generated by (3, 3)

    Z is of order 81
    What do i do about the prime power order?
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  4. #4
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    Re: Cylic

    well <(3,3)> has order 3 right? so Z9xZ9/<(3,3)> has order 27. no matter how you slice and dice it, it looks like you'll have 2 choices:

    1. Z9xZ9/<(3,3)> ≅ Z3 x Z9
    2. Z9xZ9/<(3,3)> ≅ Z3 x (Z3 x Z3).

    how can you tell which one you have? well, if you have an element of order > 3, you DON'T have the second one.

    so, tell me, what is the order of (0,1) + <(3,3)>?
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  5. #5
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    Re: Cylic

    4
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  6. #6
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    Re: Cylic

    is it?
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  7. #7
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    Re: Cylic

    you sound unconvinced, as well you should. 4 does not divide 27, so cannot be the order of any element of Z9xZ9/<(3,3)>, which has order 27. why don't you actually see what is the smallest m such that (0,m) is in <(3,3)>? you know that m ≤ 9, so that's not THAT much work.

    is (0,1) in <(3,3)>? is (0,2)? keep going...until you find the m that works.
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  8. #8
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    Re: Cylic

    1, 3
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  9. #9
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    Re: Cylic

    1,3? what does that mean?
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