My self-adjount differential operator $L$ is defined by $$L f(x) \equiv u(x) \frac{\partial^2}{\partial x^2} u(x) f(x)$$ where $u(x)$ is a known but arbitrary smooth function that satisfies $u(x) > 0$ $\forall x$.

I seek a basis of eigenfunctions of $L$, and their eigenvalues.

Are there any results known that help with this problem?

Are there any specific forms for $u(x)$ (other than $u\equiv 1$) that make the problem easier?

Edit: I am looking for eigenfunctions that do not explode, and form a useful basis in which I could in principle expand other "well behaved" functions. For example, if $u(x)\equiv 1$ I would select $e^{i\lambda x}$ as eigenfunctions, with eigenvalues $-\lambda^2$.