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Math Help - finite or infinite order of matrices

  1. #1
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    Post finite or infinite order of matrices

    I was having trouble understanding a question on my hw and was wondering if anyone could clarify. The question is:

    Let A, B be 2x2 matrices with

    A = |0 1| and B = |0 -1| and A,B elements in GL2(R)
    |-1 0| |1 -1|

    Show that A and B both have finite order but AB does not.

    I thought that to show order you take powers of the matrix until you get back to the identity and if only power that gets you to the identity is 0 then you have finite order. And the identity in GL2(R) is |1 0|
    |0 1|
    then how can you get A or B to the identity no matter what power you take A to you get either |0 1| or |0 1|
    |-1 0| |1 0|

    and there is no way to get the zeros to 1 except for taking the 0 power correct? and B raises the same issues..am i defining order right? or am i missing something?
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  2. #2
    Super Member Matt Westwood's Avatar
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    Quote Originally Posted by ChrisBickle View Post
    I was having trouble understanding a question on my hw and was wondering if anyone could clarify. The question is:

    Let A, B be 2x2 matrices with

    A = |0 1| and B = |0 -1| and A,B elements in GL2(R)
    |-1 0| |1 -1|

    Show that A and B both have finite order but AB does not.

    I thought that to show order you take powers of the matrix until you get back to the identity and if only power that gets you to the identity is 0 then you have finite order. And the identity in GL2(R) is |1 0|
    |0 1|
    then how can you get A or B to the identity no matter what power you take A to you get either |0 1| or |0 1|
    |-1 0| |1 0|

    and there is no way to get the zeros to 1 except for taking the 0 power correct? and B raises the same issues..am i defining order right? or am i missing something?
    You must be evaluating A^2 wrong, I make it:

    A^2 = \begin{matrix} -1 & 0 \\ 0 & -1 \end{matrix}. Square that and you get back to the identity.
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  3. #3
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    sorry im still confused
    |0 1|
    |-1 0|

    then A(squared) would be

    |0 1|
    |1 0|

    and the 0's are on the wrong diagonal and with B

    |0 -1|
    |1 -1|

    how would you get the -1 and 1 to 0 and the 0 to a 1?
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  4. #4
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    You have some misunderstandings.
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  5. #5
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    Oh ok its because Asquared is not square all entries in A it is matrix A x matrix A and for 3rd power etc u just multipy another one?
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  6. #6
    Super Member Matt Westwood's Avatar
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    Quote Originally Posted by ChrisBickle View Post
    Oh ok its because Asquared is not square all entries in A it is matrix A x matrix A and for 3rd power etc u just multipy another one?
    Correct.
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  7. #7
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    Quote Originally Posted by ChrisBickle View Post
    I was having trouble understanding a question on my hw and was wondering if anyone could clarify. The question is:

    Let A, B be 2x2 matrices with

    A = |0 1| and B = |0 -1| and A,B elements in GL2(R)
    |-1 0| |1 -1|

    Show that A and B both have finite order but AB does not.

    I thought that to show order you take powers of the matrix until you get back to the identity and if only power that gets you to the identity is 0 then you have finite order. And the identity in GL2(R) is |1 0|
    |0 1|
    then how can you get A or B to the identity no matter what power you take A to you get either |0 1| or |0 1|
    |-1 0| |1 0|

    and there is no way to get the zeros to 1 except for taking the 0 power correct? and B raises the same issues..am i defining order right? or am i missing something?
    I just can't understand what you say in "...I thought that to show order you take powers of the matrix until you get back to the identity and if only power that gets you to the identity is 0 then you have finite order"..???

    Anyway, multiply A by itself and get A^2 = -I ==> A^4 = I and A has order 4, and now check that B^3 = I, so ord(A) = 4 and ord(B) = 3...
    Now show that AB is the matrix

    (1 n)
    (0 1)

    and AB has infinite order.

    Tonio
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  8. #8
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    Thanks for all your help guys I really appreciate it
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