Perturbation method with triangular propagators and anharmonicities of intermediate strength

Abstract

I propose a new version of the Rayleigh - Schr\"{o}dinger perturbation method. It admits a lower triangular matrix in place of the usual diagonal propagator. Illustrated on rational anharmonicities polynomial}(x)/polynomial}(x), treated as perturbations of (quasi-)exact anharmonic oscilators. In this sense the method works in an intermediate-coupling regime and bridges the gap between the weak- and strong-coupling expansions.