@article{MM2_1980__31_154704,
     author = {Peter T. Johnstone},
     title = {Open {Maps} of {Toposes.}},
     journal = {Manuscripta mathematica},
     pages = {217--248},
     publisher = {mathdoc},
     volume = {31},
     year = {1980},
     zbl = {0433.18002},
     url = {http://geodesic.mathdoc.fr/item/MM2_1980__31_154704/}
}