Spectra self-similarity for almost Mathieu operators

Abstract

We determine numerically the self-similarity maps for spectra of the almost Mathieu operators, a two-dimensional fractal-like structure known as the Hofstadter butterfly. The similarity maps each have a horizontal component determined by certain algebraic maps, and vertical component determined by a Mobius transformation, indexed by a semigroup of the matrix group $GL_2(\Z)$. Based on the numerical evidence, we state and prove a continuity result for the similarity maps. We note a connection between the indexing of the similarity maps and Morita equivalence of rotation algebras $A_θ$, a continuous field of C*-algebras.