The Stacks project

Proof. Let $S$ be a scheme. Let $f : X \to Y$ be a morphism of algebraic spaces over $S$. Let $\{ Y_ i \to Y\} $ be an fpqc covering (Topologies on Spaces, Definition 72.9.1). Let $f_ i : X_ i \to Y_ i$ be the base change of $f$ by $Y_ i \to Y$. If $f$ is a local complete intersection morphism, then each $f_ i$ is a local complete intersection morphism by Lemma 75.48.4.

Conversely, assume each $f_ i$ is a local complete intersection morphism. We may replace the covering by a refinement (again because flat base change preserves the property of being a local complete intersection morphism). Hence we may assume $Y_ i$ is a scheme for each $i$, see Topologies on Spaces, Lemma 72.9.5. Choose a scheme $V$ and a surjective étale morphism $V \to Y$. Choose a scheme $U$ and a surjective étale morphism $U \to V \times _ Y X$. We have to show that $U \to V$ is a local complete intersection morphism of schemes. By Topologies on Spaces, Lemma 72.9.4 we have that $\{ Y_ i \times _ Y V \to V\} $ is an fpqc covering of schemes. By the case of schemes (More on Morphisms, Lemma 37.60.19) it suffices to prove the base change

of $U \to V$ by $V \times _ Y Y_ i \to V$ is a local complete intersection morphism. We can write this as the composition

The first arrow is an étale morphism of schemes (as a base change of $U \to V \times _ Y X$) and the second arrow is a local complete intersection morphism of schemes as a flat base change of $f_ i$. The result follows as being a local complete intersection morphism is syntomic local on the source and since étale morphisms are syntomic (More on Morphisms, Lemma 37.60.20 and Morphisms, Lemma 29.36.10). $\square$