- #1

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[tex]\left\langle R^{2}\right\rangle = \sum_{i,j=1}^{N} \left\langle r_{i} r_{j}\right\rangle + \left\langle \sum_{i \neq j}^{N} r_{i} r_{j}\right\rangle[/tex]

And that the cross terms drop out to produce the solution of Nb^2. However I don't understand the origin of these cross terms in the first place - what do they physically represent? And how would one evaluate the sum in the cross terms??