1. A. F. Timan, Theory of Approximation of Functions of a Real Variable [in Russian], Fizmatgiz, Moscow (1960).
2. M. P. Carrol, ?L-approximation of discontinuous functions with interpolation constraints,? J. Math. Anal. Appl.,46, No. 1, 132?142 (1974).
3. M. P. Carrol and H. W. McLaughlin, ?On L-approximation of discontinuous functions,? J. Approx. Theory,8, No. 2, 129?132 (1973).
4. I. K. Daugavet, Introduction to the Theory of Approximation of Functions [in Russian], Leningrad State Univ. (1977).
5. N. P. Korneichuk, Extremal Problems of Approximation Theory [in Russian], Nauka, Moscow (1976).