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Trace proof

  1. Oct 19, 2009 #1
    1. The problem statement, all variables and given/known data
    Show that the trace functional on n X n matrices is unique in the following
    sense. If W is the space of n X n matrices over the field F and if f is a linear functional
    on W such that f(AB) = f(BA) for each A and B in W, then f is a scalar
    multiple of the trace function. If, in addition, f(I) = n, then f is the trace function.

    3. The attempt at a solution

    I'm not sure how to start with this proof. Any help would be appreciated.
    Last edited: Oct 20, 2009
  2. jcsd
  3. Oct 19, 2009 #2


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    Any linear functional over nxn matrices has the form W(A)=C_{ij}*A_{ij} summed over i and j, right? Now work over a basis of the nxn matrices. Define D_{ij} to be the matrix with a 1 in the position {ij} and zero everywhere else. Now I'll just give you couple of examples. Let's just use 2x2 matrices. Let A=D_{12} and B=D_{11}. Then D_{12}*D_{11}=0 so W(D_{12}*D_{11})=0. D_{11}*D_{12}=D_{12}. So W(D_{11}*D_{12})=W(D_{12})=C_{12}. Hence C_{12}=0. Now look at D_{12} and D_{21}. Can you show C_{11}=C_{22}? Do you see how this is working?
    Last edited: Oct 19, 2009
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