@article{AL_1999_38_6_a1,
     author = {Yu. L. Ershov},
     title = {$\Delta$-spaces},
     journal = {Algebra i logika},
     pages = {667--679},
     publisher = {mathdoc},
     volume = {38},
     number = {6},
     year = {1999},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1999_38_6_a1/}
}