Bethe did not find the work in Frankfurt very stimulating, and in 1929 he accepted an offer from Ewald at the Technische Hochschule in Stuttgart. While there, he wrote what he considered to be his greatest paper, Zur Theorie des Durchgangs schneller Korpuskularstrahlen durch Materie ("The Theory of the Passage of Fast Corpuscular Rays Through Matter"). Starting from Max Born 's interpretation of the Schrödinger equation, Bethe produced a simplified formula for collision problems using a Fourier transform, which is known today as the Bethe formula. He submitted this paper for his habilitation in 1930.