%0 Journal Article
%A L. Hörmander
%T Differential operators of principal type
%J Matematika
%D 1961
%P 89-114
%V 5
%N 5
%I mathdoc
%U http://geodesic.mathdoc.fr/item/MAT_1961_5_5_a4/
%G ru
%F MAT_1961_5_5_a4