Voir la notice de l'article provenant de la source Theory and Applications of Categories website
It is well known how to compute the number of orbits of a group action. A related problem, apparently not in the literature, is to determine the number of elements in an orbit. The theory that addresses this question leads to orbital extensive categories and to combinatorial aspects of such categories.
@article{TAC_2015_30_a16, author = {Ernie Manes}, title = {Extensive categories and the size of an orbit}, journal = {Theory and applications of categories}, pages = {599--619}, publisher = {mathdoc}, volume = {30}, year = {2015}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_2015_30_a16/} }
Ernie Manes. Extensive categories and the size of an orbit. Theory and applications of categories, Tome 30 (2015), pp. 599-619. http://geodesic.mathdoc.fr/item/TAC_2015_30_a16/