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Residue Theorem

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The statement of the theorem

For a meromorphic function ff inside and on a closed contour γ\gamma, γf(z)dz=2πikRes(f,ak)\oint_\gamma f(z) dz = 2\pi i \sum_{k} \text{Res}(f, a_k), where the sum is over the singularities aka_k inside γ\gamma.
Source: Wikipedia