... ammann@iecn.u-nancy.frUniversit¨at Mainz, Mainz, GermanyE-mail address : lauter@mathematik.uni-mainz.dePennsylvania State University, University Park, PAE-mail address : nistor@math.psu.eduReferences[1]B. Ammann, A. ... Springer-Verlag, New York, 19 93. [37 ]C. Parenti, Operatori pseudo-differenzali inRne applicazioni, Annali Mat. Pura edApp. 93 (1972), 35 9 38 9. [38 ]E. Schrohe, Spectral invariance, ellipticity, and ... polygons, Ann. Mat. Pura Appl. 133 (19 83) , 30 5 32 6.[7]M. Crainic and R. Fernandes, Integrability of Lie brackets, Ann. of Math. 157 (20 03) ,575–620.[8]M. Dauge, Elliptic Boundary Value Problems...