1. Allgemeine Eigenwerttheorie Hermitescher Funktionaloperatoren
2. On Some General Problems of the Theory of Ordinary Differential Equations;Tamarkin,1917
3. Eigenfunction Expansions Associated with Second-Order Differential Equations. I;Titchmarsh,1964
4. Eigenfunction Expansions for Second-Order Differential Equations;Levitan,1950