Einstein's paper On the quantum theorem of Sommerfeld and Epstein [1,2], published in 1917, proposes a generalization of the quantization rule of Bohr, Sommerfeld and Epstein for multi-dimensional integrable systems. Einstein also points out that non-integrable systems could not be quantized with such rules. This observation indicates for the first time the non-triviality of the semiclassical limit of chaotic systems and can be considered a pioneer in the theory of quantum chaos.
Einstein; quantization rules; quantum chaos