Definition 75.7.2. Let $S$ be a scheme. Let $f : X \to Y$ be a morphism of algebraic spaces over $S$. The *sheaf of differentials $\Omega _{X/Y}$ of $X$ over $Y$* is sheaf of differentials (Modules on Sites, Definition 18.33.10) for the morphism of ringed topoi

of Properties of Spaces, Lemma 65.21.3. The *universal $Y$-derivation* will be denoted $\text{d}_{X/Y} : \mathcal{O}_ X \to \Omega _{X/Y}$.

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