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We define a Levi category to be a weakly orthogonal category equipped with a suitable length functor and prove two main theorems about them. First, skeletal cancellative Levi categories are precisely the categorical versions of graphs of groups with a given orientation. Second, the universal groupoid of a skeletal cancellative Levi category is the fundamental groupoid of the corresponding graph of groups. These two results can be viewed as a co-ordinate-free refinement of a classical theorem of Philip Higgins.
@article{TAC_2017_32_a22, author = {Mark V. Lawson and Alistair R. Wallis}, title = {Levi categories and graphs of groups}, journal = {Theory and applications of categories}, pages = {780--802}, publisher = {mathdoc}, volume = {32}, year = {2017}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_2017_32_a22/} }
Mark V. Lawson; Alistair R. Wallis. Levi categories and graphs of groups. Theory and applications of categories, Tome 32 (2017), pp. 780-802. http://geodesic.mathdoc.fr/item/TAC_2017_32_a22/