D. Ullrich

Divided differences and systems of nonharmonic Fourier series

David Ullrich, D. Ullrich, 1980 .

On the continuation of solutions of a certain non-linear differential equation

C. V. Coffman, D. Ullrich, 1967 .

Periodic Solutions of Second Order Nonlinear Differential Equations Without Damping

D. Ullrich, J. A. Marlin, 1968 .

Boundary value problems for a class of nonlinear second-order differential equations

D. Ullrich, 1969 .