Continuum Theory of Tkachenko Modes in Rotating Bose-Einstein Condensate
  The present paper suggests the continuum theory of Tkachenko modes in a rotating 2D Bose-Einstein condensate taking into account density inhomogeneity and compressibility of the condensate. The problem requires solution of coupled hydrodynamic equations for vortex and liquid motion with proper boundary conditions, which were derived for the condensate described by the Thomas-Fermi approximation. Compressibility becomes essential at rapid rotation with angular velocity close to the trap frequency. The theory is in a reasonable agreement with experimental observation of Tkachenko modes. 
E.B. Sonin