The Stacks project

Lemma 74.8.5. Let $S$ be a scheme. Let

\[ \xymatrix{ X \ar[rr]_ f \ar[rd]_ p & & Y \ar[dl]^ q \\ & B } \]

be a commutative diagram of morphisms of algebraic spaces over $S$. Assume that

  1. $f$ is surjective, flat, and locally of finite presentation,

  2. $p$ is syntomic.

Then both $q$ and $f$ are syntomic.

Proof. We deduce this from the analogue for schemes. Namely, the problem is ├ętale local on $B$ and $Y$ (Morphisms of Spaces, Lemma 67.36.4). Hence we may assume that $B$ and $Y$ are affine schemes. Since $|X| \to |Y|$ is open (Morphisms of Spaces, Lemma 67.30.6), we can choose an affine scheme $U$ and an ├ętale morphism $U \to X$ such that the composition $U \to Y$ is surjective. In this case the result follows from Descent, Lemma 35.14.7. $\square$

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