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Homework Help: Trace(matrix) = 0 and the dimension of subspace

  1. Nov 29, 2008 #1
    1. The problem statement, all variables and given/known data

    Claim: Matrices of trace zero for a subspace of M_n (F) of dimension n^2 -1 where M_n (F) is the set of all nxn matrices over some field F.

    2. Relevant equations

    Tr(M_n) = sum of diagonal elements

    3. The attempt at a solution

    I view the trace Tr as a linear transformation Tr: M_n (F) -> F. I find the dimension of the subspace spanned by M_n (F) by using the fact:

    dim M_n (F) = dim Tr [M_n (F)] + dim null [M_n (F)]

    Since dim M_n (F) = n^2, we have

    n^2 = dim Tr [M_n (F)] + dim null [M_n (F)]

    I'm confused on what's in the right part of the above equation:

    Isn't dim Tr [M_n (F)] = 0 since Tr [M_n (F)] = 0?

    Shouldn't dim null [M_n (F)] =n^2 - dim Tr [M_n (F)] = n^2 - 1?

    Also, I know that the subspace is generated by the following matrices: A, B, and [A,B] (the commutator of A and B).

  2. jcsd
  3. Nov 29, 2008 #2
    You said

    > Isn't dim Tr [M_n (F)] = 0 since Tr [M_n (F)] = 0?

    No, dim Tr [M_n (F)] = 1 since Tr [M_n (F)] = F
    You have confused M_n (F) (all nxn matrices over F) with its subset of trace zero matrices. And a (non degenerate) field over itself obviously has dimension 1 because it's spanned by its element 1 (multiplicative identity). So you get indeed

    dim null [M_n (F)] =n^2 - dim Tr [M_n (F)] = n^2 - 1
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