  During the interval from 1924–26, in which Weyl was intensely  occupied with the pure mathematics of Lie groups, the essentials of  the formal apparatus of the new revolutionary theory of quantum  mechanics had been completed by Heisenberg, Schrödinger and  others. As if to make up for lost time, Weyl immediately returned  from pure mathematics to theoretical physics, and applied his new  group theoretical results to quantum mechanics. As Yang (1986, 9, 10) describes it, 