1. Baider, A., Noncompact Riemannian manifolds with discrete spectra. J. Differential Geom. 14 (1979), no. 1, 41–58.
2. Collet, J.F., and Volpert, V.A., Computation of the index of linear elliptic operators in unbounded cylinders. J. Funct. Anal. 164 (1999), 34–59.
3. Dodziuk, J., L
2
-harmonic forms on rotationally symmetric Riemannian manifolds. Proc. Amer. Math. Soc. 77 (1979), no. 3, 395–401.
4. Daletskii, Yu.L., and Krein, M.G., Stability of Solutions to Differential Equations in a Banach Space. Nauka, Moscow, 1970.
5. Dezin, A.A., General Questions of the Theory of Boundary Problems. Nauka, Moscow, 1980.