A Functional Identity involving Elliptic Integrals

Abstract

We show that the following double integral \[\int_{0}^π{\rm d}x \int_0^x {\rm d}y \frac{1}{\sqrt{1-\smash[b]{p}\cos x}\sqrt{1+\smash[b]{q\cos y}}}\]remains invariant as one trades the parameters $p$ and $q$ for $p^{\prime }=\sqrt{1-p^2}$ and $q^{\prime }=\sqrt{1-q^2}$ respectively. This invariance property is suggested from symmetry considerations in the operating characterstics of a semiconductor Hall-effect device.