@book{13990,
	author = {Kaumpera, A. and Spencer, D. C. and Spencer, D. C. ; jt. auth.},
	title = {Systems of linear partial differential equations and deformation of pseudogroup structures},
	publisher = {Universite de Montreal},
	year = {1974},
	series = {University of Montreal, seminaire de Mathematiques superieures-ETE 1969},
	address = {Montreal}
}