Exact sum rules for inhomogeneous strings

TL;DR

This paper derives explicit sum rules for the eigenvalues of inhomogeneous strings, providing a diagrammatic method to estimate fundamental frequencies and improve bounds using asymptotic analysis and sequence transformations.

Contribution

It introduces a novel diagrammatic expansion for sum rules of inhomogeneous string eigenvalues and applies it to improve bounds on the fundamental mode energy.

Findings

Explicit sum rules for eigenvalues derived

Diagrammatic expansion with factorial number of diagrams

Improved bounds using asymptotic behavior and Shanks transformation

Abstract

We derive explicit expressions for the sum rules of the eigenvalues of inhomogeneous strings with arbitrary density and with different boundary conditions. We show that the sum rule of order $N$ may be obtained in terms of a diagrammatic expansion, with $(N-1)!/2$ independent diagrams. These sum rules are used to derive upper and lower bounds to the energy of the fundamental mode of an inhomogeneous string; we also show that it is possible to improve these approximations taking into account the asymptotic behaviour of the spectrum and applying the Shanks transformation to the sequence of approximations obtained to the different orders. We discuss three applications of these results.