1. Feller, W., The parabolic differential equations and the associated semi-groups
of transformations, Ann. Math., 1952, vol. 55, no. 4,
pp. 468–519.
2. Feller, W., Diffusion processes in one dimension, Trans. Am. Math. Soc., 1954, vol. 77,
pp. 1–30.
3. Shkalikov, A.A., On the basis property of eigenfunctions of ordinary
differential equations with integral boundary conditions, Moscow
Univ. Math. Bull., 1982, vol. 37, no. 6, pp. 10–20.
4. Baskakov, A.G. and Katsaran, T.K., Spectral analysis of integro-differential
operators with nonlocal boundary conditions, Differ.
Equations, 1988, vol. 24, no. 8, pp. 934–941.
5. Makin, A.S., On a nonlocal perturbation of a periodic eigenvalue problem,
Differ. Equations, 2006, vol. 42, no. 4,
pp. 599–602.