@article{HHA_1999_1_1_a1,
     author = {Ronald Brown},
     title = {Groupoids and crossed objects in algebraic topology},
     journal = {Homology, homotopy, and applications},
     pages = {1--78},
     publisher = {mathdoc},
     volume = {1},
     number = {1},
     year = {1999},
     doi = {10.4310/HHA.1999.v1.n1.a1},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4310/HHA.1999.v1.n1.a1/}
}