@article{IM2_2009_73_3_a5,
     author = {D. A. Popov},
     title = {Asymptotic behaviour of the positive spectrum of a~family of~periodic {Sturm--Liouville} problems 
},
     journal = {Izvestiya. Mathematics },
     pages = {579--610},
     publisher = {mathdoc},
     volume = {73},
     number = {3},
     year = {2009},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_2009_73_3_a5/}
}