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Math Help - On an eigenvector of matrix

  1. #1
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    On an eigenvector of matrix

    A=||A(i,j)|| (i,j=1,,n) (n>2) is a binary matrix with zero diagonal and A(i,j)=1-A(j,i) for i≠j. W=(1,1,,1) is an eigenvector for matrix B=A*A. Will W be an eigenvector for matrix A too? Why?
    Last edited by studenttt; July 18th 2012 at 11:00 AM. Reason: misprint
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  2. #2
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    Re: On an eigenvector of matrix

    Quote Originally Posted by studenttt View Post
    A=||A(i,j)|| (i,j=1,,n) is a binary matrix with zero diagonal and A(i,j)=1-A(j,i) for i≠j. W=(1,1,,1) is an eigenvector for matrix B=A*A. Will W be an eigenvector for matrix A too? Why?
    Did you try anything at all on this? In particular, what would such a 2 by 2 matrix be like? Is (1, 1)' an eigenvector in that simple case?
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    Re: On an eigenvector of matrix

    Quote Originally Posted by HallsofIvy View Post
    Did you try anything at all on this? In particular, what would such a 2 by 2 matrix be like? Is (1, 1)' an eigenvector in that simple case?
    Just forgot to specify that n>2.
    Well, here is example: A=(0,1,0; 0,0,1; 1,0,0) - is a binary matrix 3 by 3, B=A*A=(0,0,1; 1,0,0; 0,1,0) and (1, 1, 1)' is an eigenvector for B and for A too.
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  4. #4
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    Re: On an eigenvector of matrix

    But \begin{bmatrix}0 & 1 & 1 \\ 0 & 0 & 1\\ 0 & 0 & 0\end{bmatrix}\begin{bmatrix}1 \\ 1 \\ 1\end{bmatrix}= \begin{bmatrix}2 \\ 1 \\ 0\end{bmatrix} does NOT satisfy that even though the matrix does satisfy "A(i,j)=1-A(j,i) for i≠j".
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    Re: On an eigenvector of matrix

    Quote Originally Posted by HallsofIvy View Post
    But \begin{bmatrix}0 & 1 & 1 \\ 0 & 0 & 1\\ 0 & 0 & 0\end{bmatrix}\begin{bmatrix}1 \\ 1 \\ 1\end{bmatrix}= \begin{bmatrix}2 \\ 1 \\ 0\end{bmatrix} does NOT satisfy that even though the matrix does satisfy "A(i,j)=1-A(j,i) for i≠j".
    So what? Read the TS question once more - it seems to me you don't understand the question. If A=\begin{bmatrix}0 & 1 & 1 \\ 0 & 0 & 1\\ 0 & 0 & 0\end{bmatrix} it gives nothing for solution because nither A nor B=A*A will have (1,1,1)' as an eigenvector. The problem is: GIVEN that B has (1,1,...,1)' as an eigenvector. FIND (PROOVE OR UNPROOVE): Will A (always) have (1,1,...,1)' as an eigenvector? If yes - it must be proved in some way, if no - we must just find at least one matrix A (binary matrix with zero diagonal and A(i,j)=1-A(j,i) for i≠j) such that W=(1,1,,1) is not its eigenvector while W is an eigenvector for B=A*A.
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  6. #6
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    Re: On an eigenvector of matrix

    I did read the question again. You did NOT say "if (1, 1, 1, 1) was an eigenvector of B", you asserted it was. I didn't check that- I assumed you knew what you were saying.
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    Re: On an eigenvector of matrix

    Quote Originally Posted by HallsofIvy View Post
    I did read the question again. You did NOT say "if (1, 1, 1, 1) was an eigenvector of B", you asserted it was. I didn't check that- I assumed you knew what you were saying.
    It was not me who did not say "if" - it is a condition of the problem.
    It is just a common form of a problems posting (imho). Suppose the problem of a form "a and b are integer numbers. Will a+b be an integer too?". Will you say "stop.... a=1/2 is not an integer!"?. It is just a condition... every problem has got a condition (words like "if", "suppose", "assume" are meant by default) and a question.
    Our condition is "A=||A(i,j)|| (i,j=1,,n) (n>2) is a binary matrix with zero diagonal and A(i,j)=1-A(j,i) for i≠j. W=(1,1,,1) is an eigenvector for matrix B=A*A."
    That is "Suppose we have a binary matrix A=||A(i,j)|| (i,j=1,,n) (n>2) with zero diagonal and A(i,j)=1-A(j,i) for i≠j. Suppose W=(1,1,,1) is an eigenvector for matrix B=A*A." The question is "Will W (always, under the condition of the problem) be an eigenvector for matrix A too? Why?".
    By the way, if you are sure the word "if" is strictly required in the posted problem condition, I'll try to add it there.
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    Re: On an eigenvector of matrix

    Any new ideas?
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    Re: On an eigenvector of matrix

    Any new ideas, please?
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