@article{1,
  author= {Israel M. Gelfand and Ilya Zakharevich },
title= {Spectral Theory for a Pair  of Skew-symmetrical Operators on {$ S^1 $}},
journal={ Func. Anal.  Appl.},
volume={  23},
number=1,
pages={  85--93 },
year={1989}}

@article{2,
  author= {McKean, H. P. and Trubowitz, C. },
title= {Hill's Operator  and  Hyperelliptic  Function 
Theory in the Presence of Infinitely Many Branch Points  },
journal={ Comm.  Pure  Appl. Math.},
year={  1976}  ,
volume={ 29}, 
number= 1,
pages={ 143--226}}

@book{3,
  author= {Markus, A. S. },
title= {Introduction in the Spectral Theory for Polynomial  Operator Pencils },
series={Translations of Mathematical Monographs}, 
volume={71}, 
publisher={AMS},
address={  Providence}, 
year={1988} }