http://books.google.rs/books?id=vrc...e&q=Quantum theory of magnetism Ising&f=false(adsbygoogle = window.adsbygoogle || []).push({});

In page 266 why we are alowed to go from double sum to double integral?

[tex]\lim_{N\to \infty}\frac{1}{N}\sum_{q_1,q_2}F(q_1,q_2) \rightarrow \frac{1}{4\pi^2}\int^{2\pi}_0\int^{2\pi}_0dq_1dq_2F(q_1,q_2)[/tex]

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# From sum to integral

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