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

305II: Explicit prolongations of infinitesimal Lie symmetriesTable of contents1. Jet spaces and prolongations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305.2. One independent variable and one dependent variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311.3. Several independent variables and one dependent variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320.4. One independent variable and several dependent variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339.5. Several independent variables and several dependent variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348.§1. JET SPACES AND PROLONGATIONS1.1. Choice of notations for the jet space variables. Let K = R or C. Letn 1 and m 1 be two positive integers and consider two sets of variablesx = (x 1 , . . .,x n ) ∈ K n and y = (y 1 , . . .,y m ). In the classical theoryof Lie symmetries of partial differential equations, one considers certaindifferential systems whose (local) solutions should be mappings of the formy = y(x). We refer to [Ol1986] and to [BK1989] for an exposition of thefundamentals of the theory. Accordingly, the variables x are usually calledindependent, whereas the variables y are called dependent. Not to enter insubtle regularity considerations (as in [Me2005b]), we shall assume C ∞ -smoothness of all functions throughout this paper.Let κ 1 be a positive integer. For us, in a very concrete way (withoutfiber bundles), the κ-th jet space Jn,m κ consists of the space Kn+m+m(n+m)! n! m!equipped with the affine coordinates((1.2) x i , y j , y j i 1, y j i 1 ,i 2, . . ....,y j )i 1 ,i 2 ,...,i κ ,having the symmetries(1.3) y j i 1 ,i 2 ,...,i λ= y j i σ(1) ,i σ(2) ,...,i σ(λ),for every λ with 1 λ κ and for every permutation σ of the set{1, . . ., λ}. The variable y j i 1 ,i 2 ,...,i λis an independent coordinate correspondingto the λ-th partial derivativeλ y j. So the symmetries (1.3) are∂∂x i 1∂x i 2···∂x i λnatural.In the classical Lie theory ([OL1979], [Ol1986], [BK1989]), all the geometricobjects: point transformations, vector fields, etc., are local, defined ina neighborhood of some point lying in some affine space K N . However, inthis paper, the original geometric motivations are rapidly forgotten in orderto focus on combinatorial considerations. Thus, to simplify the presentation,we shall not introduce any special notation to speak of certain local