Homotopy Exact Sequence for the Pro-Étale Fundamental Group

Abstract

The pro-\'etale fundamental group of a scheme, introduced by Bhatt and Scholze, generalizes the usual \'etale fundamental group $\pi_1^{\mathrm{et}}$ defined in SGA1 and leads to an interesting class of "geometric coverings" of schemes, generalizing finite \'etale covers. We prove exactness of the general homotopy sequence for the pro-\'etale fundamental group, i.e. that for a geometric point $\bar{s}$ on $S$ and a flat proper morphism $X \rightarrow S$ of finite presentation whose geometric fibres are connected and reduced, the sequence $$ \pi_1^{\mathrm{proet}}(X_{\bar{s}}) \rightarrow \pi_1^{\mathrm{proet}}(X) \rightarrow \pi_1^{\mathrm{proet}}(S) \rightarrow 1 $$ is "nearly exact". This generalizes a theorem of Grothendieck from finite \'etale covers to geometric coverings. We achieve the proof by constructing an infinite (i.e. non-quasi-compact) analogue of the Stein factorization in this setting.