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Mathematical Methods for Physics and Engineering - Matematica.NET

Mathematical Methods for Physics and Engineering - Matematica.NET

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25.8 APPROXIMATIONS TO INTEGRALSwhich we already know has the value √ π when z is real. This choice of demonstrationmodel is not accidental, but is motivated by the fact that, as we havealready shown, in the neighbourhood of a saddle point all exponential integr<strong>and</strong>scan be approximated by a Gaussian function of this <strong>for</strong>m.The same integral can also be thought of as an integral in the complex plane,in which the integration contour happens to be along the real axis. Since theintegr<strong>and</strong> is analytic, the contour could be distorted into any other that had thesame end-points, z = −∞ <strong>and</strong> z =+∞, both on the real axis.As a particular possibility, we consider an arc of a circle of radius R centredon z =0.Itiseasilyshownthatcos2θ ≥ 1+4θ/π <strong>for</strong> −π/4

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