@article{CTGDC_2010__51_2_83_0,
     author = {Vitale, Enrico M.},
     title = {Bipullbacks and calculus of fractions},
     journal = {Cahiers de Topologie et G\'eom\'etrie Diff\'erentielle Cat\'egoriques},
     eid = {1},
     pages = {83--113},
     publisher = {Andr\'ee CHARLES EHRESMANN},
     volume = {51},
     number = {2},
     year = {2010},
     mrnumber = {2667978},
     zbl = {1226.18006},
     language = {en},
     url = {http://archive.numdam.org/item/CTGDC_2010__51_2_83_0/}
}