1. The theorems for regular integral equations may easily be carried over to cases in which fewer assumptions are made about the kernel. The definitions of regular and singular integral equations used here follow those in Ph. Frank and R. v. Mises: Differential- und Integralgleichungen der Mechanik und Physik, 2nd ed., Vol. 1, p. 535. Brunswick 1930.
2. See, for instance, Ph. Frank and R. v. Mises: (see last footnote). — Courant, R:, and D. Hilbert: Methods of mathematical physics, 1st English ed., Vol.1. New York: Interscience Publishers, Inc. 1953
3. Hamel, G.: Integralgleichungen, 2nd ed. Berlin 1949
4. Schmeidler, W.: Integralgleichungen mit Anwendungen in Physik und Technik, Vol. I, Lineare Integralgleichungen, 611 pp. Leipzig 1950.
5. See, for instance, W. Schmeidler: (see last footnote) pp. 328–360.