Expansion in the "distance" from Hc2(T) line for the mixed state of BCS   superconductors
  We develop a description of the mixed state of type II superconductivity valid within a wide range of temperatures and external magnetic fields. It is based on the quasiclassical version of microscopic BCS theory and employs an expansion in the "distance" from the Hc2(T) line. We prove that in a clean metal with spherical Fermi surface and isotropic pairing interaction the superconducting condensation always produces a hexagonal vortex lattice. 
Anton Knigavko and Frank Marsiglio