@article{INTD_1977_8_a2,
     author = {V. P. Maslov and I. A. Shishmarev},
     title = {$T$-product of hypoelliptic operators},
     journal = {Itogi Nauki i Tekhniki. Seriya Sovremennye Problemy Matematiki. Noveishie Dostizheniya},
     pages = {137--197},
     year = {1977},
     volume = {8},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/INTD_1977_8_a2/}
}