1. Aissen, M., A class of superadditive functions,Proc. Amer. Math. Soc. 4 (1951) 360?362
2. Choquet, G., Theory of capacities,Ann. Inst. Fourier (Grenoble) 5 (1953?54) 131?295
3. Hopf, E., Elementare Bemerkungen über die Lösungen partieller Differentialgleichungen zweiter Ordnung vom elliptischen Typus,Sitzungsber. Preuss. Akad. Wiss. 19 (1927) 147?152
4. Hopf, E., A remark on linear elliptic differential equations of second order,Proc. Amer. Math. Soc. 3 (1952) 791?793
5. Payne, L. E.,Some isoperimetric inequalities in the torsion problem for multiply connected regions, Studies in Mathematical Analysis and Related Topics, Essays in honor of G. Pólya, Stanford Univ. Press, Stanford, California (1962) 270?280