A Dirichlet Type Result for Ordinary Differential Operators. W.N. Everitt; M. Giertz — 1973 Mathematische Annalen
Some Remarks on a Separation and Limit - point Criterion of Second-order, Ordinary Differential Expressions. W.N. Everitt; M. Giertz; J. Weidmann — 1973 Mathematische Annalen