@article{MZM_1972_12_3_a6,
     author = {V. S. Fedii},
     title = {An example of a second-order nonhypoelliptic operator with the property of global hypoellipticity},
     journal = {Matemati\v{c}eskie zametki},
     pages = {269--274},
     publisher = {mathdoc},
     volume = {12},
     number = {3},
     year = {1972},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1972_12_3_a6/}
}