1. K. Friedrichs, On differential operators in Hilbert spaces, Amer. J. Math. vol. 61 (1939).
2. K. Friedrichs, On hyperbolic differential operators, Bull. Amer. Math. Soc. Abstract 46-9-416 (1940) p. 754.
3. M. Bôcher, On harmonic functions in two dimensions, Proceedings of the American Academy of Sciences vol. 41 (1905–1906).
4. G. C. Evans, On the reduction of integro-differential equations, Trans. Amer. Math. Soc. vol. 15 (1914) p. 485. Note on a theorem of Bôcher, Amer. J. Math. vol. 50 (1928) pp. 123–126.
5. G. C. Evans, Fundamental points of potential theory, Rice Institute Pamphlets vol. 7 (1920) p. 274. Complements of potential theory. I, Amer. J. Math. vol. 55 (1933) p. 42.