Pseudo‐Rotations versus Rotations

Abstract

Continuing the study of Hamiltonian pseudo-rotations of projective spaces, we focus on the conjecture that the fixed-point data set (the actions and the linearized flows at one-periodic orbits) of a pseudo-rotation exactly matches that data for a suitable unique true rotation even though the two maps can have very different dynamics. We prove this conjecture in several instances, for example, for strongly non-degenerate pseudo-rotations of C P 2 ${\mathbb {C}}{\mathbb {P}}^2$ with some notable exceptions, which we call ghost pseudo-rotations. The existence of ghost pseudo-rotations is a completely open question. The conjecture is closely related to the properties of the action and index spectra of pseudo-rotations, and ghost pseudo-rotations, if they exist, satisfy all known restrictions on the fixed-point data for pseudo-rotations but these data are distinctly different from the data for any rotation. The main new ingredient of the proofs is purely combinatorial and of independent interest. This is the index divisibility theorem connecting the divisibility properties of the Conley–Zehnder index sequence for the iterates of a map with the properties of its spectrum.