文摘
For an algebraic stack \fancyscriptX{\fancyscript{X}} flat and of finite presentation over a scheme S, we introduce various notions of relative connected components and relative irreducible components. The main distinction between these notions is whether we require the total space of a relative component to be open or closed in \fancyscriptX{\fancyscript{X}}. We study the representability of the associated functors of relative components, and give an application to the moduli stack of curves of genus g admitting an action of a fixed finite group G.