J. Hounie et Ep. Delemos, SOLVABILITY IN L(P) OF FIRST-ORDER LINEAR-OPERATORS, Journal of mathematical analysis and applications, 197(1), 1996, pp. 42-53
We prove that first-order partial differential operators of principal
type with smooth coefficients are locally solvable in L(p), 1 < P < in
finity, if they satisfy condition (P). (C) 1996 Academic Press, Inc.