On the best approximation of the differentiation operator on the half-line

We solve a problem of S. B. Stechkin concerning the best approximation in the metric of C to the operator of k-th order differentiation on certain classes of differentiable functions defined on the half-line, by linear operators whose norms from L₂ into C are bounded. We consider the analogous problem for linear differential operators with constant coefficients. The bibliography contains 10 items.Translated from Matematicheskie Zametki, Vol. 6, No. 5, pp. 573–582, November, 1969.