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Math Help - Sylow question

  1. #1
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    Sylow question

    Assume that G is a group of order 63 that has two Sylow subgroups whose intersecion is non-trivial. Show that G has an element of order 21.
    So by Sylow's theroem, I know that 63=3^2*7, that the sylow 7 subgroup is a normal subgroup of G with 6 order 7 elements. My sylow 3 subgroup is of order 9 and there can be one or 2 sylow three subgroups. Since I know that s7 (sylow 7 subgroup) is normal, I was thinking about trying to use this fact ot generate a cylic subgroup of order 21. Am I anywhere on the right path?
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  2. #2
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    Quote Originally Posted by wutang View Post
    Assume that G is a group of order 63 that has two Sylow subgroups whose intersecion is non-trivial. Show that G has an element of order 21.
    So by Sylow's theroem, I know that 63=3^2*7, that the sylow 7 subgroup is a normal subgroup of G with 6 order 7 elements. My sylow 3 subgroup is of order 9 and there can be one or 2 sylow three subgroups.


    No. There can only be 1 or 7 Sylow 3-subgroups


    Since I know that s7 (sylow 7 subgroup) is normal, I was thinking about trying to use this fact ot generate a cylic subgroup of order 21. Am I anywhere on the right path?

    I think you are: now you only need to produce elements x,y\in G\,\,\,s.t.\,\,\,ord(x)=7\,,\,ord(y)=3\,,\,xy=yx to deduce that ord(xy)=21 ...I'd check some non-trivial element in the intersection of two Sylow 3-subgroups.

    Tonio
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  3. #3
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    I'd check some non-trivial element in the intersection of two Sylow 3-subgroups.
    So do I have to priove that their are 7 Sylow 3-subgroups, and from this I would get a subgroup of order 21, and since 7 is a prime and 3 is a prime they would both be in that subgroup? I am unsure how this implies that their would be an order 21 element though...
    How would I prove that there is 7 3-sylows?
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  4. #4
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    So since we have two sylow subgroups whose intersection is non-trival, ths means that we have 7 sylow 3 subgroups. This means we have 14 elements of order 3 in are group G. Since we have a unique sylow 7 subgroup, we have 6 elements of order 7 in G.
    Let H = sylow 7 subgroup.
    So for every a in G, aha^-1 is a subgroup of order 7, so we must have aHa^-1=H (since H is normal). So N(H) = G (N(H)= normalizer of H). SInce H has prime order, it is cyclic and abelian. So C(H) contains H (C(H)= centralizer of H in G). So 7 divides |C(H)| and |C(H)| divides 63. So C(H)= H or C(H)=G, or |C(H)|= 21.
    Is this enough to get that xy=yx wth |x,y|=21?
    Thanks again for your help.
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  5. #5
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    Quote Originally Posted by wutang View Post
    I'd check some non-trivial element in the intersection of two Sylow 3-subgroups.
    So do I have to priove that their are 7 Sylow 3-subgroups, and from this I would get a subgroup of order 21, and since 7 is a prime and 3 is a prime they would both be in that subgroup? I am unsure how this implies that their would be an order 21 element though...
    How would I prove that there is 7 3-sylows?

    If there's only one Sylow 3-sbgp. then it is normal, and since also the Sylow 7-sbgp. is normal and (9,7) = 1 , we get that G is the direct product of these two Sylow sbgps., which are each abelian, and thus G is abelian, and a finite abelian group has a subgroup of order any divisor of its order, and thus there's a sbgp. of G of order 21 which automatically is cyclic...

    So the non-trivial case is when there's more than 1 Sylow 3-sbgp.

    Tonio
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