Finite difference operators with a finite--band spectrum
  We study the correspondence between almost periodic difference operators and algebraic curves (spectral surfaces). An especial role plays the parametrization of the spectral curves in terms of, so called, branching divisors. The multiplication operator by the covering map with respect to the natural basis in the Hardy space on the surface is the $2d+1$--diagonal matrix; the $d$--root of the product of the Green functions (counting their multiplicities) with respect to all infinite points on the surface is the symbol of the shift operator. We demonstrate an application of our general construction to a particular covering, which generate widely discussed almost periodic CMV matrices. We discuss an important theme: covering of one spectral surface by another one and related to this operation transformations on the set of multidiagonal operators (so called Renormalization Equations). We proof several new results dealing with Renormalization Equations for periodic Jacobi matrices (polynomial coverings) and the case of a rational double covering. 
F. Peherstorfer, and P. Yuditskii