Best approximation of unbounded operators by bounded ones (Q1921703)

This is a survey on best approximation of unbounded operators by bounded ones with 127 items in the references, including: 1. Relation of the Stechkin problem with other extremal problems; 2. Existence of best approximating operators and their properties; 3. Optimal approximation of functionals; 4. The best approximation of operators of differentiation and inequalities between norms of derivatives; 5. Approximation of invariant operators.