1. Keselman, G.M.: On the unconditional convergence of eigenfunction expansions of certain differential operators. Izv. Vyssh. Uchebn. Zaved. Mat. 2, 82–93 (1964)
2. Mikhailov, V.P.: On Riesz basis in $$L_2(0, 1)$$. Dokl. Akad. Nauk SSSR. 144(5), 981–984 (1962)
3. Dunford, N., Schwartz, J.T.: Linear Operators. Part III, Spectral Operators. Wiley, New York (1971)
4. Ionkin, N.I., Moisseev, E.I.: A problem for the heat equation with two-point boundary conditions. Differ. Equ. 15, 1284–1295 (1979)
5. Sadybekov, M.A.: Initial-boundary value problem for a heat equation with not strongly regular boundary conditions. In: Functional Analysis in Interdisciplinary Applications. Springer Proceedings in Mathematics $$\&$$ Statistics, vol. 216, pp. 330–348. Springer, Berlin (2017)