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The Cauchy Residue Theorem in Complex Analysis

The Cauchy Residue Theorem states that if f is analytic on a domain D and its boundary except at finitely many isolated singularities interior to D, then the contour integral of f over the boundary equals 2πi times the sum of the residues of f at those singularities. This belongs to complex analysis and builds directly on the theory of Laurent series and residues, generalizing the Cauchy Integral Formula to regions containing multiple singularities, and it forms the foundation for later techniques that evaluate real-valued integrals using complex-analytic contour methods.