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I don't understand this

  1. Sep 16, 2009 #1
    [tex]\frac{1}{2}S\sum_{\vec{n},\vec{m}}I_{\vec{n}-\vec{m}}[(S-S_{\vec{n}}^z)+(S-S_{\vec{m}}^z)]=SI(0)\sum_{\vec{m}}(S-S_{\vec{m}}^z)[/tex]

    How can I get this result?
     
  2. jcsd
  3. Sep 16, 2009 #2
    Separate the terms into two different sums, then you will have one sum that looks like [tex]\sum_{n,m} I_{n-m} (S-S^z_n)[/tex] and another with S^z_m. For the former, the sum over m can be carried out.
     
  4. Sep 16, 2009 #3
    What does this equation describe (just curious)?
     
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