International Congress of Mathematicians

712 N. Lerner1. From Hans Lewy to Nirenberg-Treves'condition (^)Year 1957.The Hans Lewy operator L 0 , introduced in [20], is the following complex vectorfield in R 3 d d dL Q = - Vi- \-i(xi+ix 2 )^—• (1.1)OXi OX2 OX3There exists / £ C°° such that the equationL 0 u = f (1.2)has no distribution solution, even locally. This discovery came as a great shock forseveral reasons. First of all, L 0 has a very simple expression and is natural as theCauchy-Riemann operator on the boundary of the pseudo-convex domain{(zi,z 2 ) £ C 2 ,|zi| 2 + 2Im0 2 < 0}.Moreover L 0 is a non-vanishing vector field so that no pathological behaviour relatedto multiple characteristics is to be expected. In the fifties, it was certainly theconventional wisdom that any "reasonable" operator should be locally solvable, andobviously (1.1) was indeed very reasonable so the conclusion was that, once more,the CW should be revisited. One of the questions posed by such a counterexamplewas to find some geometric explanation for this phenomenon.1960.This was done in 1960 by L.Hörmander in [7] who proved that if p is thesymbol of a differential operator such that, at some point (x, £) in the cotangentbundle,p(x.fi) = 0 and {Rep,lmp}(x.fi) > 0, (1.3)then the operator P with principal symbol p is not locally solvable at x; in fact,there exists / £ C°° such that, for any neighborhood V of x the equation Pu = fhas no solution u £ V(V). Of course, in the case of differential operators, thesign > 0 in (1.3) can be replaced by ^ 0 since the Poisson bracket {Rep, Imp} isthen an homogeneous polynomial with odd degree in the variable £. Nevertheless,it appeared later (in [8]) that the same statement is true for pseudo-differentialoperators, so we keep it that way. Since the symbol of -iL 0 is £1 —X2^3+i(^2+xi^),and the Poisson bracket {£1 —X2Ç3, £2 + Ï1C3} = 2£3, the assumption (1.3) is fulfilledfor L 0 at any point x in the base and the nonsolvability property follows. This givesa necessary condition for local solvability of pseudo-differential equations: a locallysolvableoperator P with principal symbol p should satisfy{Rep, lmp}(x, £) < 0 at p(x.fi) = 0. (1.4)Naturally, condition (1.4) is far from being sufficient for solvability (see e.g. thenonsolvable Af 3 below in (1.5)). After the papers [20], [7], the curiosity of the