Structure and classification of monoidal groupoids
  The structure of monoidal categories in which every arrow is invertible is analyzed in this paper, where we develop a 3-dimensional Schreier-Grothendieck theory of non-abelian factor sets for their classification. In particular, we state and prove precise classification theorems for those monoidal groupoids whose isotropy groups are all abelian, as well as for their homomorphisms, by means of Leech's cohomology groups of monoids. 
Mar\'ia Calvo, Antonio M. Cegarra, Benjam\'in A. Heredia
math.CT math.KT