Theory of Multi-gap Superfluidity Based on Fermi-liquid Approach
  Superfluid-to-superfluid phase transitions in a Fermi-liquid leading to the emergence of two-gap superfluid states have been studied.   There are considered the systems of fermions of one and two (nuclear matter) sorts. The self-consistent equations describing new two-gap superfluid states have been obtained. These equations differ essentially from the equations of the BCS theory. The solutions of the self-consistent equations corresponding to one-gap and two-gap superfluid states have been found. 
A. I. Akhiezer, A. A. Isayev, S. V. Peletminsky, A. A. Yatsenko