A relative monotone-light factorisation system for internal groupoids

Abstract

Given an exact category $\mathcal{C}$, it is well known that the connected component reflector $\pi_0\colon\mathsf{Gpd}(\mathcal{C})\to\mathcal{C}$ from the category $\mathsf{Gpd}(\mathcal{C})$ of internal groupoids in $\mathcal{C}$ to the base category $\mathcal{C}$ is semi-left-exact. In this article we investigate the existence of a monotone-light factorisation system associated with this reflector. We show that, in general, there is no monotone-light factorisation system $(\mathcal{E}',\mathcal{M}^*)$ in $\mathsf{Gpd}(\mathcal{C})$, where $\mathcal{M}^*$ is the class of coverings in the sense of the corresponding Galois theory. However, when restricting to the case where $\mathcal{C}$ is an exact Mal'tsev category, we show that the so-called comprehensive factorization of regular epimorphisms in $\mathsf{Gpd}(\mathcal{C})$ is the relative monotone-light factorisation system (in the sense of Chikhladze) in the category $\mathsf{Gpd}(\mathcal{C})$ corresponding to the connected component reflector, where $\mathcal{E}'$ is the class of final functors and $\mathcal{M}^*$ the class of regular epimorphic discrete fibrations.