The Birkhoff Normal Form through the Lens of Representation Theory
摘要
We derive the simplest version of the finite-order Birkhoff normal forms [BNFs], that for area-preserving maps of the plane,using the finite-dimensional representation theory for the group of linear area-preserving maps of that plane and of its circle subgroup. We describe our motivation: the utility of understanding the 3rd-order BNF to obtain KAM stability for non-trivialperiodic orbits which arise in celestial mechanics.