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3. I. I. Ibragimov and A. D. Gadzhiev, ?A certain sequence of positive linear operators,? Dokl. Akad. Nauk SSSR,193, No. 6, 1222?1225 (1970).
4. A. D. Gadzhiev, ?k-Positive linear operators in the space of regular functions and theorems of Korovkin type,? Izv. Akad. Nauk Az. SSR, Ser. Fiz.-Mat. Tekhn. Nauk, No. 5, 49?53 (1974).