1. N. K. Bari, “Biorthogonal systems and bases in Hilbert space,” in Uchenye Zapiski Matematika (Izd. Moskov. Univ., 1951), Vol. 148, pp. 69–107 [in Russian].
2. I. Schur, “Über endliche Gruppen und Hermitesche Formen,” Math. Z. 1(2–3), 184–207 (1918).
3. H. Rademacher, “Einige Sätzeüber Reihen von allgemeinen Orthogonalfunktionen,” Math. Ann. 87(1–2), 112–138 (1922).
4. E.M. Nikishin, “Convergence of certain series of functions,” Izv. Akad. Nauk SSSR Ser. Mat. 31(1), 15–26 (1967) [Math. USSR-Izv. 1, 13–23 (1968)].
5. S. Kachmazh and H. Steinhaus, Theorie der Orthogonalreihen (Warszawa-Lwow, 1935; Fizmatgiz, Moscow, 1958).