Approximation theory : proceedings of the conference jointly organized by the Mathematical Institute of the Polish Academy of Sciences and the Institute of Mathematics of the Adam Mickiewicz University, held in Poznan, 22-26 August, 1972 - Ciesielski, Zbigniew, 1934-; Musielak, Julian, 1928- (PWN-Polish Scientific Publishers ; D. Reidel, 1975)