@article{ZNSL_2002_285_a14,
     author = {S. P. Khekalo},
     title = {Isohuygens deformation of the ultrahyperbolic operator},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {207--223},
     year = {2002},
     volume = {285},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_2002_285_a14/}
}