@article{10_4064_fm205_2_1,
     author = {Valentin Gutev and Vesko Valov},
     title = {Open maps having the {Bula} property},
     journal = {Fundamenta Mathematicae},
     pages = {91--104},
     publisher = {mathdoc},
     volume = {205},
     number = {2},
     year = {2009},
     doi = {10.4064/fm205-2-1},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/fm205-2-1/}
}