Reference Categories structures [ehresmann-dblcats]

 @article
{ehresmann-dblcats, title={Categories structures}, volume={80}, ISSN={0012-9593}, number={4}, journal={Annales scientifiques de l'Ecole Normale Superieure}, publisher={Elsevier}, author={Ehresmann, Charles}, year={1963}, pages={349-426},language={fra} }