The Cauchy Principal Value and the finite part integral as values of absolutely convergent integrals

Abstract

The divergent integral ${\int }_a^{b}f(x)(x-x_0{)}^{-n-1}\mathrm{d}x$, for $-\infty <a<x_0<b<\infty$ and $n=0,1,2,\dots$, is assigned, under certain conditions, the value equal to the simple average of the contour integrals ${\int }_{C^{\pm }}f(z)(z-x_0{)}^{-n-1}\mathrm{d}z$, where $C^{+}$ ($C^{-}$) is a path that starts from $a$ and ends at $b$, and which passes above (below) the pole at $x_0$. It is shown that this value, which we refer to as the Analytic Principal Value, is equal to the Cauchy principal value for $n=0$ and to the finite-part of the divergent integral for positive integer $n$. This implies that, where the conditions apply, the Cauchy principal value and the finite-part integral are in fact values of absolutely convergent integrals. Moreover, it leads to the replacement of the boundary values in the Sokhotski-Plemelj-Fox Theorem with integrals along some arbitrary paths. The utility of the Analytic Principal Value in the numerical, analytical and asymptotic evaluation of the Cauchy principal value and the finite-part integral is discussed and demonstrated.

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