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A Note on a Sum of Lognormals

This note considers the applicability of Gauss-Hermite quadrature and direct numerical quadrature for computation of moment generating function (mgf) and the derivatives. A preprocessing using the asymptotic technique is employed while computing the characteristic function (chf) using Gauss Hermite quadrature while this is optional for mgf. The mgf of the low and high amplitude regions of a single lognormal variable and the derivatives is examined and attention is drawn to the effect of variance. The problem of inversion of the mgf/chf of a sum of lognormals to obtain the CDF/pdf is considered with special reference to methods related to Post Widder technique, Gaussian quadrature and the Fourier series method. The method based on the complex exponential integral which makes use of the derivative of the cumulant is an alternative. Segmentation of the mgf/chf on the basis of the derivative structure which indicates activity rate is shown to be useful.