TY  - JOUR
AU  - Mlček, Josef
TI  - Approximations of $\sigma$-classes and $\pi$-classes
JO  - Commentationes Mathematicae Universitatis Carolinae
PY  - 1979
SP  - 669
EP  - 679
VL  - 20
IS  - 4
PB  - mathdoc
UR  - http://geodesic.mathdoc.fr/item/CMUC_1979__20_4_a4/
LA  - en
ID  - CMUC_1979__20_4_a4
ER  -