1. 1. J. C. F. Sturm, J. de Math. 1 (1836), 106.
2. 2. Mauro Picone, Ann. Scuola Norm. Sup. Pisa 11 (1909), 1.
3. 3. Maxime Bôcher, Boundary problems in one dimension, Proc. 5th Int. Congress Math., I, Cambridge, 1912, pp. 163-195.
4. 4. Maxime Bôcher, Leçons sur les methodes de Strum, Gauthier-Villars, Paris, 1917.