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3. V. V. Arestov, Best approximation of translation invariant unbounded operators by bounded linear operators, Tr. Mat. Inst. Akad. Nauk, 198 (1992), 3–20 (in Russian); translated in Proc. Steklov Inst. Math., 198 (1994), 1–16.
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5. V. V. Arestov, On the best approximation of the differentiation operator, Ural Math. J., 1 (2015), 20–29.