Take a complex logarithm whose radial branch cut has angle . On a positively oriented circle, use the one-sided branch values on either side of its crossing with the cut. Parametrize from to ; adding an integer multiple of to the branch does not change the integral because the integral of is zero. Along the open arc, a primitive is . Its endpoint difference is times the cut-crossing point , proving the displayed formula. The circle crosses the cut, so this is a piecewise contour integral of boundary values, not the integral of a single holomorphic logarithm on a neighborhood of a closed circle. The Cauchy integral theorem therefore does not make it zero.
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