# Convergent perturbation expansion of energy eigenfunctions on unperturbed basis states in classically-forbidden regions

## TL;DR

This paper develops a convergent perturbation expansion for energy eigenfunctions in classically-forbidden regions, based on their properties in classically-allowed regions, applicable across all perturbation strengths.

## Contribution

It introduces a novel convergent perturbation expansion for eigenfunctions in forbidden regions, extending perturbation theory beyond small perturbations.

## Key findings

- Perturbation expansion converges in the forbidden region.
- Expansion valid for all perturbation strengths.
- Provides a link between allowed and forbidden region components.

## Abstract

We study properties of eigenfunctions of perturbed systems, given on the eigenbases of unperturbed, integrable systems. For a given pair of perturbed and unperturbed systems, with respect to the energy of each perturbed state, the unperturbed basis states can be divided into two groups: one in the classically-allowed region and the other in the classically-forbidden region; correspondingly, the eigenfunction of the perturbed state is also divided into two parts. In the semiclassical limit, it is shown that, making use of components of the eigenfunction in its classically-allowed region, its components in the classically-forbidden region can be written in the form of a convergent perturbation expansion, which is valid for all perturbation strengths.