“…Indeed, several papers are devoted to the interplay between the signature of the Levi form and the possibility of locally solving the system d u = f , possibly in a smaller neighborhood; see Lewy [13], Hörmander [11], Andreotti and Hill [2], Andreotti, Fredricks, and Nacinovich [1], Nacinovich [15,16,17], Kashiwara and Schapira [12], Michel [14], Treves [20,21,22,23], Chen and Shaw [9], Peloso and Ricci [19], Hill and Nacinovich [10], Nicola [18]. We also refer, for other invariants, to the contributions by Cordaro and Hounie [4,5,6], Cordaro and Treves [7,8], Chanillo and Treves [3].…”