He had written his thesis under Courant 's supervision and he then became Courant 's assistant for two years helping with the Courant - Hilbert book. He collaborated with Lewy on linear hyperbolic partial differential equations and they wrote a joint paper in 1927, and another joint paper, with Courant and Lewy, considered the stability of difference schemes for partial differential equations.
Niya ang kanyang mga sanaysay na nakasulat sa ilalim ng Courant 's pangangasiwa at pagkatapos ay siya naging Courant' s assistant para sa dalawang taon na ang pagtulong sa Courant - Hilbert libro. He collaborated sa Lewy sa guhit hayperboliko bahagyang kaugalian equation at sila wrote pinagsamang papel sa 1927, at iba pang pinagsamang papel, sa Courant at Lewy, isinasaalang-alang ang katatagan ng pagkakaiba na pamamaraan para sa mga bahagi ng kaugalian equation.