Einstein and the old quantum theory

Abstract

Objecting that Sommerfeld's quantum conditions refer to particular coordinates, Einstein proposes a canonically invariant rule. But even if the invariance is canonical, Einstein may have in mind a double \emph{configuration} space invariance: with respect to loop deformations, and to \emph{point} transformation -- all on a torus where features of the Liouville-Arnol'd theorem already appear.