Travaux sur les symétries de Lie des équations aux ... - DMA - Ens

192or equivalently, which has a power series expansion of the form:Θ ′( z ′ ,z ′ ,w ′) = − w ′ +∑∑ ∑Θ ′ α,β,0 z′ α z′β + w ′ γΘ ′ α,β,γ z′ α z′β .α1, β1γ1α1, β1Levi nondegenerate hypersurfaces. Leaving aside the real defining equationof M, let us now rename the complex defining equation of M in suchnormalized coordinates simply as before: w = Θ(z, z, w), dropping all theprime signs. Quite concretely, the real analytic hypersurface M is said to beLevi nondegenerate at the origin if the coefficient Θ 1,1,0 of zz, which may bechecked to always be real because of the reality condition (7.28), is nonzero.In fact, it is well known that Levi nondegeneracy is a biholomorphically invariantproperty, see for instance [18], p. 158, but in more conceptual terms,the following general characterization, which may be taken as a definitionhere, holds true. One then readily checks that it is equivalent to Θ 1,1,0 ≠ 0in normalized coordinates.Lemma. ([Me2005a, Me2005b, 19]) The real analytic hypersurface M ⊂C 2 with 0 ∈ M represented in coordinates (z, w) by a complex definingequation of the form w = Θ(z, z, w) is Levi nondegenerate at the origin ifand only if the map:(z,w)↦−→(Θ(0, z, w), Θz (0, z, w) )has nonvanishing 2 × 2 Jacobian determinant at (z, w) = (0, 0).After a possible real dilation of the z-coordinate, we can therefore assumethat Θ 1,1,0 = 1, and then we are provided with the following normalization:(7.28) w = −w + zz + zz O ( |z| + |w| ) ,that will be useful shortly. Another, even more convincing argument forconsigning to oblivion the real defining equation u = ϕ(x, y, v) dates backto Beniamino Segre [23], who observed that to any real analytic M are associatedtwo deeply linked objects.1) The nowadays so-called Segre varieties 15 S q associated to any pointq ∈ C 2 near the origin of coordinates (z q , w q ) that are the complexcurves defined by the equation:S q := { 0 = −w + Θ ( z, z q , w q)},quite appropriately in terms of the fundamental complex defining functionΘ; this equation is holomorphic just because its antiholomorphicterms are set fixed.15 A presentation of the general theory, valuable for generic CR manifolds of arbitrarycodimension d 1 and of arbitrary CR dimension m 1 in C m+d enjoying no specificnondegeneracy condition, may be found in [Me2005a, Me2005b, 18].