1. Agmon, S. and Nirenberg, L., Properties of Solutions of Ordinary Differential Equations in Banach Space, Comm. Pure Appl. Math., 1963, vol. 16, no. 2, pp. 121–239.
2. Agmon, S., Douglis, A., and Nirenberg, L., Otsenki reshenii ellipticheskikh uravnenii vblizi granitsy (Estimates for Solutions of Elliptic Equations near the Boundary), Moscow, 1962.
3. Nikishkin, V.A., Singularities of the Solution of the Dirichlet Problem for a Second-Order Equation in a Neighborhood of an Edge, Tr. Mosk. Mat. Obs., 1981, vol. 42, pp. 243–253.
4. Agranovich, M.S. and Vishik, M.I., Elliptic Problems with a Parameter and Parabolic Problems of General Form, Uspekhi Mat. Nauk, 1964, vol. 19, no. 3, pp. 53–161.
5. Gilbarg, D. and Trudinger, N., Elliptic Partial Differential Equations of Second Order, Berlin: Springer-Verlag, 1983. Translated under the title Ellipticheskie differentsial’nye uravneniya s chastnymi proizvodnymi vtorogo poryadka, Moscow: Nauka, 1989.