Boundaries in digital planes. Khalimsky, Efim; Kopperman, Ralph; Meyer, Paul R. — 1990 Journal of Applied Mathematics and Stochastic Analysis
Natural continuity space structures on dual Heyting algebras Marcel Erné; Ralph Kopperman — 1990 Fundamenta Mathematicae
Cardinal invariants of bitopological spaces Ralph D. Kopperman; Paul R. Meyer — 1989 Časopis pro pěstování matematiky