1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Diagonalisation of matrices

  1. Sep 21, 2005 #1
    Hi I'm wondering if the 'order' in which vectors are taken is important in the process of matrix diagonalisation. To clarify what I mean here is an example.

    A = \left[ {\begin{array}{*{20}c}
    7 & { - 2} \\
    {15} & { - 4} \\
    \end{array}} \right]

    I need to diagonalise matrix A. So I need a matrix D such that [tex]D = P^{ - 1} AP[/tex].

    I calculate the eigenvalues for A, and got bases for the eigenspace associated with each of the eigenvalues. Following the procedure in my book I took the union of the two(it turned out that there are two bases) bases which I found to be: {(2,5),(1,3)}.

    So [tex]P = \left[ {\begin{array}{*{20}c}
    2 & 1 \\
    5 & 3 \\
    \end{array}} \right] \Rightarrow P^{ - 1} = \left[ {\begin{array}{*{20}c}
    3 & { - 1} \\
    { - 5} & 2 \\
    \end{array}} \right][/tex] where I have formed the matrix P whose columns are the vectors in the set which is the union of the two bases for the eigenspaces.

    My calculations yield [tex]D = \left[ {\begin{array}{*{20}c}
    2 & 0 \\
    0 & 1 \\
    \end{array}} \right][/tex].

    The answer is [tex]D = \left[ {\begin{array}{*{20}c}
    1 & 0 \\
    0 & 2 \\
    \end{array}} \right][/tex].

    I'm not sure where my error is. I've checked the matrix multiplication for D and also PP^-1 = I.
  2. jcsd
  3. Sep 21, 2005 #2

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    there is no canonical diagonal form. if the basis of e-vectors is u,v for you, then in the book they've simply got v,u in the reverse order.

    they differ by a poermutaion of the basis (try multiplying yours by the matrix with 1's on the off diagonal places and zero on the diagonals (a self inverse symmetric matrix)
  4. Sep 21, 2005 #3


    User Avatar
    Homework Helper

    The eigenvalues appear in the diagonal matrix in the same order as the corresponding eigenvectors in the columns the P-matrix.
  5. Sep 21, 2005 #4
    Thanks for the help matt grime and TD.
  6. Jul 2, 2010 #5
    1 2
    3 5
    you'll get the desired answer within a moment.This change does'nt alter the physical meaning,but the form only.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook