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Homework Help: Find the complete orthonormal set of eigenfunctions of the operators B-hat

  1. Oct 16, 2011 #1
    1. The problem statement, all variables and given/known data

    A bound quantum system has a complete set of orthonormal, no-degenerate energy eigenfunctions u(subscript n) with difference energy eigenvalues E(subscript n). The operator B-hat corresponds to some other observable and is such that:

    B u(subscript 1)=u(subscript 2)
    B u(subscript 2)=u(subscript 1)
    B u(subscript n)=0
    n>3 or B=3

    a) Find the complete orthonormal set of eigenfunctions of the operator B-hat (expand out the eigenvalues of B in terms of u, and do not neglect any solutions)
    b) If B is measured and found to have the eigenvalue H, what is the expectation value of the energy in the resulting state?

    3. The attempt at a solution

    B u(subscript1)=u(subscript2)
    B u(subscript 2)=u(subscript1)
    (B^2) u*(subscript 2)=u(subscript2)
    B^2 =1

    I don't think this is leading anywhere. Please help.
  2. jcsd
  3. Oct 16, 2011 #2


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    Remember B can be represented by a matrix relative to the {un} basis. Once you have the matrix, you can diagonalize it to find the eigenvalues of B.
  4. Oct 17, 2011 #3
    weirdly I can't edit the original post. I meant 'non-degenerate' rather than no-degenerate
  5. Oct 17, 2011 #4
    I don't know how to represent B as a matrix. I tried looking it up but the u(n) thing is confusing me. How does B relate to u(n)?
  6. Oct 22, 2011 #5
    B|t(subscript n)>=b(subscript n) |t(subscript n)>

    |t(subscript n)>=sigma a(subscript i) |u(subscript i)>

    |b(subscript i)>=sigma c(subscript n)|u(subscript n>

    B-hat|b(i)>=sigma c(n) B-hat|u(n)>
    B-hat|b(i)>=c(1) u(1)+c(2)u(1)=b|b(n)>

    b c(1)=c(2)
    b c(2)=c(1)


    n> or=3 b c(n)=0

    c(n)=0, n>or=3


    <b(2)|b(2)>=c^2 (<u(1)|u(1)>+<u(2)|u(2)>)



    I don't think I have answered the question completely though.

    I have no idea how to find the expectation value of the energy. Please help
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