Non-Self-Adjoint Hill Operators whose Spectrum is a Real Interval

Let $H = -d^2/dx^2 + q(x)$, $x \in \mathbb{R}$, where $q(x)$ is a periodic potential, and suppose that the spectrum $\sigma(H)$ of $H$ is the positive semi-axis $[0, \infty)$. In the case where $q(x)$ is real-valued (and locally square-integrable) a well-known result of G. Borg states that $q(x)$ must vanish almost everywhere. However, as it was first observed by M.G. Gasymov, there is an abundance of complex-valued potentials for which $\sigma(H) = [0, \infty)$. In this article we conjecture a characterization of all complex-valued entire potentials whose spectrum is $[0, \infty)$. We also present an analog of Borg's result for complex potentials.