@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/