Pullback and pushout squares in a special double category with connection
Cahiers de topologie et géométrie différentielle, Tome 24 (1983) no. 2, pp. 161-192. 
@article{CTGDC_1983__24_2_161_0,
     author = {Spencer, C. B. and Wong, Y. L.},
     title = {Pullback and pushout squares in a special double category with connection},
     journal = {Cahiers de topologie et g\'eom\'etrie diff\'erentielle},
     pages = {161--192},
     publisher = {Dunod \'editeur, publi\'e avec le concours du CNRS},
     volume = {24},
     number = {2},
     year = {1983},
     mrnumber = {710039},
     zbl = {0519.18008},
     language = {en},
     url = {http://www.numdam.org/item/CTGDC_1983__24_2_161_0/}
}
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Spencer, C. B.; Wong, Y. L. Pullback and pushout squares in a special double category with connection. Cahiers de topologie et géométrie différentielle, Tome 24 (1983) no. 2, pp. 161-192. http://www.numdam.org/item/CTGDC_1983__24_2_161_0/

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