A summation formula over the zeros of a combination of the associated Legendre functions with a physical application

TL;DR

This paper derives a summation formula for zeros of combined associated Legendre functions using the Abel-Plana formula and applies it to evaluate the Wightman function in curved space with spherical boundaries.

Contribution

It introduces a new summation formula for zeros of combined Legendre functions and applies it to quantum field theory in curved space with spherical boundaries.

Findings

Derived a summation formula for zeros of combined Legendre functions.

Expressed the Wightman function as a sum of two integrals for spherical shell geometry.

Analyzed boundary-induced effects on vacuum expectation values.

Abstract

By using the generalized Abel-Plana formula, we derive a summation formula for the series over the zeros of a combination of the associated Legendre functions with respect to the degree. The summation formula for the series over the zeros of the combination of the Bessel functions, previously discussed in the literature, is obtained as a limiting case. As an application we evaluate the Wightman function for a scalar field with general curvature coupling parameter in the region between concentric spherical shells on background of constant negative curvature space. For the Dirichlet boundary conditions the corresponding mode-sum contains series over the zeros of the combination of the associated Legendre functions. The application of the summation formula allows us to present the Wightman function in the form of the sum of two integrals. The first one corresponds to the Wightman function for the geometry of a single spherical shell and the second one is induced by the presence of the second shell. The boundary-induced part in the vacuum expectation value of the field squared is investigated. For points away from the boundaries the corresponding renormalization procedure is reduced to that for the boundary-free part.