Lee has made contributions to the existence, uniqueness, and continuous dependence theory for solutions to nonlinear boundary value problems. This work, which continues, is joint with R.B. Guenther of Oregon State and A. Granas of the University of Montreal. Lee has also worked on the numerical calculation of solutions to such problems. He has helped develop the extension of Sturm oscillation theory and the properties of Sturm-Liouville eigenvalue problems to higher order equations. This work is closely related to the branch of approximation theory which deals with Tchebycheff systems and with positive operator theory. It led to related work on best quadrature formulas.