His work on this topic occupied him for the last 25 years of his life and he published much of it in Eigenfunction Expansions Associated with Second-Order Differential Equations (1946, 1958).
Kanyang trabaho sa paksang ito ginagawa sa kanya para sa huling 25 taon ng kanyang buhay at siya nai-publish na marami ng mga ito sa Eigenfunction Expansions Associated sa Ikalawang-Order kaugalian equation (1946, 1958).