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: I don't understand this

  1. Sep 16, 2009 #1

    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)?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook