Lemma 97.6.1. Let p : \mathcal{X} \to \mathcal{Y} and q : \mathcal{Z} \to \mathcal{Y} be 1-morphisms of categories fibred in groupoids over (\mathit{Sch}/S)_{fppf}. If p : \mathcal{X} \to \mathcal{Y} is formally smooth on objects, then so is the base change p' : \mathcal{X} \times _\mathcal {Y} \mathcal{Z} \to \mathcal{Z} of p by q.
Proof. This is formal. Let U \subset U' be a first order thickening of affine schemes over S, let z' be an object of \mathcal{Z} over U', let w be an object of \mathcal{X} \times _\mathcal {Y} \mathcal{Z} over U, and let \delta : p'(w) \to z'|_ U be an isomorphism. We may write w = (U, x, z, \alpha ) for some object x of \mathcal{X} over U and object z of \mathcal{Z} over U and isomorphism \alpha : p(x) \to q(z). Note that p'(w) = z hence \delta : z \to z|_ U. Set y' = q(z') and \gamma = q(\delta ) \circ \alpha : p(x) \to y'|_ U. As p is formally smooth on objects there exists an object x' of \mathcal{X} over U' as well as isomorphisms \beta : x'|_ U \to x and \gamma ' : p(x') \to y' such that (97.6.0.1) commutes. Then we consider the object w = (U', x', z', \gamma ') of \mathcal{X} \times _\mathcal {Y} \mathcal{Z} over U' and define isomorphisms
and
These combine to give a solution to the problem. \square
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