Some Inequalities Associated with Certain Ordinary Differential Operators. William N. Everitt; Magnus Giertz — 1972 Mathematische Zeitschrift
On the limit-3 classification of the square of a second-order, linear differential expression William Norrie Everitt; Magnus Giertz — 1976 Czechoslovak Mathematical Journal
On limit-point and separation criteria for linear differential expressions Everitt, William Norrie; Giertz, Magnus — 1973 Proceedings of Equadiff III