On a Comparison Theorem for Self-Adjoint Partial Differential Equations of Elliptic Type
Philip E. Hartman, Aurel Wintner · Proceedings of the American Mathematical Society · 1955
Philip Hartman, Aurel Wintner, On a Comparison Theorem for Self-Adjoint Partial Differential Equations of Elliptic Type, Proceedings of the American Mathematical Society, Vol. 6, No. 6 (Dec., 1955), pp. 862-865