@article{TMF_1985_62_2_a2,
     author = {N. E. Firsova},
     title = {Levinson formula for perturbed {Hill} operator},
     journal = {Teoreti\v{c}eska\^a i matemati\v{c}eska\^a fizika},
     pages = {196--209},
     year = {1985},
     volume = {62},
     number = {2},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/TMF_1985_62_2_a2/}
}