@article{FAA_1975_9_4_a4,
     author = {V. F. Lazutkin and T. F. Pankratova},
     title = {Normal forms and versal deformations for {Hill's} equation},
     journal = {Funkcionalʹnyj analiz i ego prilo\v{z}eni\^a},
     pages = {41--48},
     publisher = {mathdoc},
     volume = {9},
     number = {4},
     year = {1975},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/FAA_1975_9_4_a4/}
}