Best approximation, Chebyshev systems, Dual spaces of operator algebras, 510.mathematics, Abstract approximation theory (approximation in normed linear spaces and other abstract spaces), Linear spaces of operators, Norms (inequalities, more than one norm, etc.) of linear operators, Article, Riesz operators; eigenvalue distributions; approximation numbers, \(s\)-numbers, Kolmogorov numbers, entropy numbers, etc. of operators