@article{RM_1991_46_3_a2,
     author = {N. Kh. Ibragimov and A. O. Oganesyan},
     title = {The hierarchy of {Huygens} equations in spaces with a~non-trivial conformal group},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     pages = {137--176},
     publisher = {mathdoc},
     volume = {46},
     number = {3},
     year = {1991},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/RM_1991_46_3_a2/}
}