@article{RM_1986_41_4_a5,
     author = {E. I. Ganzha},
     title = {Hypoellipticity of a second-order differential operator with quadratic form of variable sign},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     pages = {175--176},
     publisher = {mathdoc},
     volume = {41},
     number = {4},
     year = {1986},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/RM_1986_41_4_a5/}
}