@article{CMUC_1979__20_4_a4,
     author = {Ml\v{c}ek, Josef},
     title = {Approximations of $\sigma$-classes and $\pi$-classes},
     journal = {Commentationes Mathematicae Universitatis Carolinae},
     pages = {669--679},
     publisher = {mathdoc},
     volume = {20},
     number = {4},
     year = {1979},
     mrnumber = {555182},
     zbl = {0449.03051},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CMUC_1979__20_4_a4/}
}