The Spectral Asymptotics of the Two-Dimensional Schr\"odinger operator   with a Strong Magnetic Field
  We consider the spectral problem for the two-dimensional Schr\"odinger operator for a charged particle in strong uniform magnetic and periodic electric fields. The related classical problem is analyzed first by means of the Krylov-Bogoljubov-Alfven and Neishtadt averaging methods. It allows us to show ``almost integrability'' of the the original two-dimensional classical Hamilton system, and to reduce it to a one-dimensional one on the phase space which is a two-dimensional torus. Using the topological methods for integrable Hamiltonian system and elementary facts from the Morse theory, we give a general classification of the classical motion. According this classification the classical motion is separated into different regimes with different topological characteristics (like rotation numbers and Maslov indices). Using these regimes, the semiclassical approximation, the Bohr-Sommerfeld rule and the correspondence principle, we give a general asymptotic description of the (band) spectrum of the original Schr\"odinger operator and, in particular, estimation for the number of subbands in each Landau band. From this point of view the regimes, are the classical preimages of ``spectral series'' of the Schr\"odinger operator. We also discuss the relationship between this spectrum and the spectrum of one-dimensional difference operators. 
Jochen Bruening, Sergey Dobrokhotov, Konstantin Pankrashkin