- #1

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_{i}S

_{j}S

_{k}S

_{l}> and the indices i,j,k and l, given a Hamiltonian H.

Solution:

The problem is that i can't follow the last two steps in this given solution. I understand how the partition function Z is obtained and how they express the four spin average as the trace of the four spins and the "density operator" (e^(-beta*H)/Z). But im very confused about how they continue to the next last and last rows.

I'm thankful for explanations!