%0 Journal Article
%A Mlček, Josef
%T Approximations of $\sigma$-classes and $\pi$-classes
%J Commentationes Mathematicae Universitatis Carolinae
%D 1979
%P 669-679
%V 20
%N 4
%I mathdoc
%U http://geodesic.mathdoc.fr/item/CMUC_1979__20_4_a4/
%G en
%F CMUC_1979__20_4_a4