DefinitionThe residual theorem is a powerful method in complex analysis through which the value of the contour integral needs to be calculated. That is, if f is analytic inside and on a simple closed contour C, possibly excluding isolated singularities, then the integral of f around C is equal to 2πi times the sum of the residues of f at these singularities. Example: Consider ∮Cz2+1ezdz, where C is the unit circle.