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On the Korteweg-de Vries equation: Convergent Birkhoff normal form. (English) Zbl 0868.35099
The authors consider the Korteweg-de Vries (KdV) equation $V_t+ V_{xxx}- 6V\cdot V_x=0$ on the circle, i.e., for functions $$V=V(x,t)$$ on $$S^1\times\mathbb{R}$$. It is well-known that the KdV equation represents a completely integrable Hamiltonian system (of infinite dimension) with phase space $$H^n(S^1;\mathbb{R})$$, where $$n$$ is a positive integer, and with a Poisson structure inducing a symplectic structure on the phase space.
The main result of the present paper states that there is a symplectic morphism from the zero-leaf of the phase space, with respect to the Casimir function defined by the Poisson structure to some symplectic Hilbert space. This symplectomorphism is bijective, real-analytic in both directions, and induces globally defined real-analytic action-angle coordinates for the entire KdV hierarchy. This theorem provides the (apparently) first infinite-dimensional counterpart of the result by J. Vey [Am. J. Math. 100, 591-614 (1978; Zbl 0384.58012)], which states that a finite-dimensional completely integrable Hamiltonian system with real-analytic Hamiltonian functional admits action-angle variables in a neighborhood of a non-resonant elliptic fixed point.
In the second part of the paper, the authors apply their main result to the study of the regularity of the KdV-Hamiltonian vector field. In particular, the local existence of a convergent Birkhoff normal form is proved.

##### MSC:
 35Q53 KdV equations (Korteweg-de Vries equations) 37J35 Completely integrable finite-dimensional Hamiltonian systems, integration methods, integrability tests 37K10 Completely integrable infinite-dimensional Hamiltonian and Lagrangian systems, integration methods, integrability tests, integrable hierarchies (KdV, KP, Toda, etc.) 37J99 Dynamical aspects of finite-dimensional Hamiltonian and Lagrangian systems
Zbl 0384.58012
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