@article{SM_1976_28_4_a6,
     author = {V. I. Feigin},
     title = {Asymptotic distribution of eigenvalues for hypoelliptic systems in~$R^n$},
     journal = {Sbornik. Mathematics},
     pages = {533--552},
     year = {1976},
     volume = {28},
     number = {4},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1976_28_4_a6/}
}