Lectures in differential equations - Aziz, A. K; Wightman, A. S; Friedrichs, K. O; Phillips, R. S; Kato, Tosio; Agmon, Shmuel; Kac, M; Getoor, R. K; McKean, H. P; Birkhoff, Garrett; Davis, Philip J; Lax, Peter D (Van Nostrand Reinhold Company, 1969)