A note on discrete Borg-type theorems

Abstract

We consider the discrete versions of the well-known Borg’s theorem and use simple linear algebraic techniques to obtain new versions of the discrete Borg-type theorems. To be precise, we prove that the periodic potential of a discrete Schrödinger operator is almost a constant if and only if the possible spectral gaps of the operator are of small width. This result is further extended to more general settings and the connection to the well-known Ten Martini problem is also discussed.