1. S. B. Stechkin, ?The absolute convergence of orthogonal series,? Matem. Sb.,29, 225?232 (1951).
2. S. B. Stechkin, ?The absolute convergence of orthogonal series,? Dokl. Akad. Nauk SSSR,102, 37?40 (1955).
3. G. Freud, Orthogonale Polynome, Berlin (1969).
4. G. Freud, ?Über die absolute Konvergenz von orthogonalen Polynomreihen,? Acta Math. Acad. Sci. Hung.,4, 127?135 (1953).
5. S. Kachmazh and G. Shteingauz, The Theory of Orthogonal Series [in Russian], Moscow (1958).