@article{10_4064_fm_114_3_219_228,
     author = {Marek Wilhelm},
     title = {Criteria of openness for relations},
     journal = {Fundamenta Mathematicae},
     pages = {219--228},
     publisher = {mathdoc},
     volume = {114},
     number = {3},
     year = {1981},
     doi = {10.4064/fm-114-3-219-228},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/fm-114-3-219-228/}
}