  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.
