1. Bari, N.K., Biorthogonal systems and bases in a Hilbert space, Matematika (Mathematics), Moscow, 1951, vol. 4: Uch. Zap. Moskov. Gos. Univ., no. 148, pp. 69–107.
2. Kessel’man, G.M., On unconditional convergence of decompositions in eigenfunctions of some differential operators, Izv. Vyssh. Uchebn. Zaved. Mat., 1964, no. 2, pp. 82–93.
3. Mikhailov, V.P., On Riesz bases in L2(0, 1), Dokl. Akad. Nauk SSSR, 1962, vol. 144, no. 5, pp. 981–984.
4. Shkalikov, A.A., On the basis problem of the eigenfunctions of an ordinary differential operator, Russ. Math. Surveys, 1979, vol. 34, no. 5 (209), pp. 249–250.
5. Makin, A.S., On many-point spectral boundary value problems, Differ. Equations, 2000, vol. 36, no. 10, pp. 1461–1468.