@article{CTGDC_1977__18_4_409_0, author = {Spencer, Christopher B.}, title = {An abstract setting for homotopy pushouts and pullbacks}, journal = {Cahiers de topologie et g\'eom\'etrie diff\'erentielle}, pages = {409--429}, publisher = {Dunod \'editeur, publi\'e avec le concours du CNRS}, volume = {18}, number = {4}, year = {1977}, zbl = {0378.18008}, mrnumber = {486054}, language = {en}, url = {http://www.numdam.org/item/CTGDC_1977__18_4_409_0/} }