1. Courant, R., and D. Hilbert, Methods of Mathematical Physics, Vol. II, Interscience Publishers, New York, 1962.
2. Bers, L., Isolated Singularities of Minimal Surfaces, Ann. of Math.,531951), pp 364–386; see also Singularities of Minimal Surfaces,Proc. Int. Cong. Math., 1950,2 (1952), pp. 157–164.
3. Bers, L.,Function-theoretical Properties of Solutions of Partial Differential Equations of Elliptic Type,Annals of Mathematics Study, No. 33, 1954.
4. Finn, R., Isolated Singularities of Solutions of Non-linear Partial Differential Equations,Trans. Amer. Math. Soc.,751953), pp. 385–404.
5. Finn, R., On partial Differential Equations (whose solutions admit no isolated singularities),Scripta Mathematica, Vol. XXVI, No. 2, 1961.