Asymptotic evaluation of multidimensional oscillatory integrals

Abstract: Oscillatory integrals appear throughout physics, from diffraction and gravitational lensing to real-time path integrals. Their asymptotic evaluation is complicated by the need to determine which real and complex saddle points contribute, and how they are connected by Stokes phenomena. In multiple dimensions, this traditionally requires computing steepest descent manifolds in a high-dimensional complex space. Here, we develop an algebraic alternative for integrals with polynomial and rational exponents. Using integration by parts and Brieskorn reduction, we construct a finite-dimensional Gauss-Manin system that generates the asymptotic expansion of all saddle points. Its Borel transform yields a Fuchsian system, whose mondromy determines the Stokes constants and hence the adjacency of the saddle points. Combined with the recent South-East rule, this identifies the contributing saddles and enables superasymptotic and hyperasymptotic approximations without computing gradient flows. We demonstate the method for the parabolic umbilic diffraction integral and a Lorentzian lens as examples, obtaining agreement to machine precision. The methods are implemented in an accompanying open-source Julia package Resurgence.jl