Polynomial normal forms for ODEs near a center-saddle equilibrium point

Abstract: In this work we consider a saddle-center equilibrium for general vector fields as well as Hamiltonian systems, and we transform it locally into a polynomial normal form in the saddle variables by a change of coordinates. This problem was first solved by Bronstein and Kopanskii in 1995, as well as by Banyaga, de la Llave and Wayne in 1996 [BLW] in the saddle case. The proof used relies on the deformation method used in [BLW], which in particular implies the preservation of the symplectic form for a Hamiltonian system, although our proof is different and, we believe, simpler. We also show that if the system has sign-symmetry, then the transformation can be chosen so that it also has sign-symmetry. This issue is important in our study of shadowing non-transverse heteroclinic chains (Delshams and Zgliczynski 2018 and 2024) for the toy model systems (TMS) of the cubic defocusing nonlinear Schr\"odinger equation (NLSE) on $2D$-torus or similar Hamiltonian PDE, which are used to prove energy transfer in these PDE.