Split Lemma and First Isomorphism Theorem for groupoids

Abstract: Groupoids are the oidification of groups, and they are largely used in topology and representation theory. We consider here the category $\mathsf{Gpd}$ of all groupoids with all morphisms, and the category $\mathsf{Gpd}\Lambda$ of groupoids over a fixed set of vertices $\Lambda$, with morphisms fixing $\Lambda$. Famously, the First Isomorphism Theorem fails to hold in $\mathsf{Gpd}$. However, we retrieve here a universally lifted version of the First Isomorphism Theorem in $\mathsf{Gpd}$, through the definition of virtual kernels. In $\mathsf{Gpd}\Lambda$ instead, a First Isomorphism Theorem is already known from \'Avila, Mar\'in, and Pinedo (2020). Semidirect products of a group by a groupoid are well known. We define crossed products in $\mathsf{Gpd}$, and prove that they are equivalent to split epimorphisms. We observe that in $\mathsf{Gpd}\Lambda$ crossed products and semidirect products are essentially equivalent, under mild assumptions, and our Split Lemma in $\mathsf{Gpd}$ collapses to a much simpler Split Lemma in $\mathsf{Gpd}\Lambda$. This latter one, in turn, under some mild extra assumptions, implies a Split Lemma by Ibort and Marmo (2023).