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

148nondegeneracy. Let l 0 ∈ N with l 0 ≥ 1. We shall assume that M is l 0 -finitely nondegenerate at the origin, cf. [BER1999], [Me2003], [8]. Thismeans that there exist multiindices β(1), . . ., β(n) ∈ N n with |β(k)| ≥ 1for k = 1, . . .,n and max 1≤k≤n |β(k)| = l 0 , and integers j(1), . . .,j(n)with 1 ≤ j(k) ≤ m for k = 1, . . .,n such that the local holomorphic mapping(7.28) (C n+m ∋ (¯z, ¯w) ↦−→ (Θ j (0, ¯z, ¯w)) 1≤j≤m , ( Θ j(k),z β(k)(0, ¯z, ¯w) ) )∈ C m+n1≤k≤nis of rank equal to n + m at (¯z, ¯w) = (0, 0). Here, we denote the partialderivative ∂ |β| Θ j (0, ¯z, ¯w)/∂z β simply by Θ j,z β(0, ¯z, ¯w). Then M is Levinondegenerate at the origin if and only if l 0 = 1. By complexifying thevariables ¯z and ¯w, we get new independent variables ζ ∈ C n and ξ ∈ C mtogether with a complex algebraic or analytic m-codimensional submanifoldM in C 2(n+m) of equations(7.28) w j = Θ j (z, ζ, ξ), j = 1, . . ., m,called the extrinsic complexification of M. In the defining equations (7.28)of M , following [Se1931] and [24], we may consider the “dependentvariables” w 1 , . . .,w m as algebraic or analytic functions of the “independentvariables” z = (z 1 , . . ., z n ), with additional dependence on the extra“parameters” (ζ, ξ) ∈ C n+m . Then by applying the differential operator∂ |α| /∂z α to (7.28), we obtain w j,z α(z) = Θ j,z α(z, ζ, ξ). Writing these equationsfor (j, α) = (j(k), β(k)) with k = 1, . . .,n, we obtain a system ofm + n equations{w j (z) = Θ j (z, ζ, ξ), j = 1, . . ., m,(7.28)w j(k),z β(k)(z) = Θ j(k),z β(k)(z, ζ, ξ), k = 1, . . ., n.In this system (7.28), by the assumption of l 0 -finite nondegeneracy (7.28),the algebraic or analytic implicit function theorem allows to solve the parameters(ζ, ξ) in terms of the variables (z k , w j (z), w j(k),z β(k)(z)), providinga local algebraic or analytic C n+m -valued mapping R such that (ζ, ξ) =R ( z k , w j (z), w j(k),z β(k)(z) ) . Finally, for every pair (j, α) different from(1, 0), . . ., (m, 0), (j(1), β(1)), . . ., (j(n), β(n)), we may replace (ζ, ξ) byR in the differentiated expression w j,z α(z) = Θ j,z α(z, ζ, ξ). This yields(w j,z α(z) = Θ j,z α z, R(zk , w j (z), w j(k),z β(k)(z)) )(7.28)(=: F j,α zk , w j (z), w j(k),z β(k)(z) ) .This is the system of partial differential equations associated with M . Asargued by B. Segre in [Se1931], the geometric study of generic submanifoldsof C n may gain much information from the study of their associatedsystems of partial differential equations (cf. [24], [25]). The next paragraphs