From the time he undertook research as a postgraduate with Birkhoff he was directed to the major problems which had interested Poincaré such as the three body problem.
Mula sa oras na siya undertook pananaliksik bilang isang postgradweyt sa Birkhoff siya ang sa malaking problema na kung saan nagkaroon na interesado Poincaré tulad ng tatlong katawan problema.