@article{GDZPPN002314185, title = {The Essential Spectrum of Elliptic Systems of Mixed Order.}, journal = {Mathematische Annalen}, volume = {229}, year = {1977}, pages = {247-276}, zbl = {0361.35050}, url = {http://dml.mathdoc.fr/item/GDZPPN002314185} }
Grubb, Gerd; Geymonat, Giuseppe. The Essential Spectrum of Elliptic Systems of Mixed Order.. Mathematische Annalen, Tome 229 (1977) pp. 247-276. http://gdmltest.u-ga.fr/item/GDZPPN002314185/