... to other than hydrogen-like atoms Einstein delivers a lecture at May 11, 1917 in Berlin - his only contribution on the energy quantization of mechanical systems ... and it will be forgotten soon.

35 years later, Keller discovers a somewhat more general quantization, but he assumes at first (in contrast to Einstein) its universal applicability. Fritz Reiche, the successor at Einstein's chair (after his emigration) at the Berlin University calls Keller's attention to the 1917 paper, and the latter draws it from oblivion.

Further twenty years later physics recalls Poincaré as well who had discovered, already around 1900, the chaotic behaviour of planetary orbits.