# On eigenproblem for inverted harmonic oscillators

## TL;DR

This paper thoroughly analyzes the eigenproblem of the inverted harmonic oscillator, revealing a complete spectrum description, eigenfunctions via hypergeometric functions, and establishing the reality of the spectrum through two mathematical approaches.

## Contribution

It provides a comprehensive spectral analysis of the inverted harmonic oscillator, including eigenfunction characterization and spectrum reality proof using unitary transformations and rigged Hilbert spaces.

## Key findings

- Eigenfunctions expressed with confluent hypergeometric functions
- Spectrum is continuous and complex in general
- Real spectrum identified for physically relevant states

## Abstract

We consider an eigenvalue problem for an inverted one dimensional harmonic oscillator. We find a complete description for the eigenproblem in $C^{\infty}(\mathbb R)$. The eigenfunctions are described in terms of the confluent hypergeometric functions, the spectrum is ${\mathbb C}$. The spectrum of the differential operator $-{\frac{d}{dx^2}}-{\omega}^{2}{x^2}$ is continuous and has physical significance only for the states which are in $L^{2}(\mathbb R)$ and correspond to real eigenvalues. To identify them we use two approaches. First we define a unitary operator between $L^{2}(\mathbb R)$ and $L^{2}$ for two copies of $\mathbb R$. This operator has the property that the spectrum of the image of the inverted harmonic oscillator corresponds to the spectrum of the operator $-i{\frac{d}{dx}}$. This shows that the (generalized) spectrum of the inverted harmonic operator is real. The second approach uses rigged Hilbert spaces.