Duality and lower bounds for relative projection constants (Q1890588)

This paper discusses the use of a duality theorem for the computation of lower bounds for relative projection constants. A first application yields a slight improvement of a lower bound which has already been proved by the author [J. Approximation Theory 62, No. 2, 206-212 (1994; Zbl 0734.41034)]. Further applications concern polynomial projections in the \(L_1\)-space as well as polynomial projections in the multivariate case. The derived lower bounds are asymptotically best possible.