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# SN~ (~6) lff 2It 3k_~ , 5 ",,x_J {b)

SN~ (~6) lff 2It 3k_~ , 5 ",,x_J {b)

## 270 then f is

270 then f is k-TCz~-determined (in the space \$ff ). (ii) The deformation F: (R n x R",O) ~ R off is Ti+2-versal if and only iyR{f'l,... ,~'a} spans £Z /J_(f), where Fi = OF/Oui(x,O). [] ]~,XAMPLE. Let f = x 5 + yS. Then J_(f) = \$~.{z4,y4}, so Thus f is 5-determined (w.r.t. 7~z2-equivalence). 4 Two degrees of freedom This section essentia~y reproduces the result of J.B. Delos [7] using generating families; the main purpose is to illustrate the technical difficulties which axise in 3 degrees of freedom. We have the following set up: L C T*Q is a Lagrangian subma.nifold invariant under the involution r : T*Q -* T*Q (defined above) with the point q = (q, 0) e L, and L gnq Q. The projection re : L ~ Q is Z2-invariant, and so has rank 0 at q. By section 2, there is an odd generating family F : R 2 x R 2 --+ R whose associated Lagrmagian projection Irv : C(F) --+ R 2 is Z2-Lagranglan equivalent to re. The organizing centre f of F is also odd. Proposition 4 Any Z2-stable Lagrangian map germ re : R 2 --* R 2 is Lagrangian equivalent to one of the following two germs: re+(=,y) = (=:,y2), re_(x,y) = (x2 _ y2,2xy). PROOF: As r is Z2-stable, F must be an odd Z2-versal family, so the organizing centre f must have Z2-codimension at most 2. Up to T~z2-equivalence, the only odd functions with this property are f.t.(x,y ) = 1(.,.g3 Jl- y3), f_(=,y) = ~=3__=y2. This follows from the usual approach to classifying critical points (see e.g. [4]), using the determinacy estimate in Proposition 3. A 12-versal deformation of f~- is given by r~(x, y,,,,v) = f~(~, y) - ,,= - vy. The result follows. [] Remark 5 Of these two Lagrangian Z2-stable singularities, only r+ can occur in the projection of invariazat Lagrangian submanifolds in classical Hamiltonian systems. The reason is as follows. Let H(q,p) = K(p) + V(q) be the Hamiltonian, where K is the kinetic energy, which is a positive definite homogeneous quadratic fimction in p, and V is the potential energy. The invariant submanifold L lies on an energy level H = E, for some E E R, and suppose that q = (q,0) E L. Since K is positive definite, the image

271 Figure 1: Two trajectories in a time reversible 2 degree of freedom Hamiltonian (the Henon-Heiles Hamiltonian). The figure on the left has 4 corners, while the one on the right has only folds and cusps: it is not a time reversible torus. of the projection of the submanifold must lie in the 'Hill's region' {q E Q 1 V(q) < E}. Moreover, q is a regular point of the boundary of this Hill's region as otherwise (q, 0) would be an equilibrium point of the system and there would not be an invariant Lagrangian submanifold through it. Thus the image of the torus lies on one side of a smooth curve through q. The map 7r_ does not have this property, only the map r+ does. This leaves open the question of whether a r_ singularity can occur stably in a nonclassical system. It is natural to ask what degeneracies of projections of time reversible Lagrangian submanifolds might occur, after all in 2 degrees of freedom one might expect to have a 2 parameter family of invariant tori. However, such a degeneracy would require there to be a ~r_ singularity, either in a transition from a r+ to a r_ or as a coalescing of a r+ and a r_. In either case there is the probably non-physical r_, so we do not pursue this. The sort of degeneracy that is more likely to occur is that the Lagrangian submanifold (torus) itself becomes singular, for example along an invariant submanifold of lower dimension such as a periodic orbit. 5 Three degrees of freedom We begin by mimicking the constructions for 2 degrees of freedom. The Lagrangian projection will be defined by an odd generating family whose organizing centre ] is a critical point of corank 3 (so without loss of generality, an odd function of 3 variables). Thus f is a homogeneous ternary cubic plus (possibly) higher order terms. After a suitable linear choice of coordinates, any real nondegenerate homogeneous ternary cubic can be written in the form, f~(x, ~, z) = x ~ + 2cx2z + =z2 + y~. (z) Of these, only f~l are degenerate. This expression is known as the Legendre normal form for a non-degenerate ternary cubic. (A homogeneous polynomial is said to be non- degenerate if the origin is the only critical point over C.)

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