The Stacks project

Lemma 105.2.4. In Situation 105.2.1 let $x_0 : \mathop{\mathrm{Spec}}(k) \to \mathcal{X}$ be a morphism, where $k$ is a finite type field over $S$. Then a versal ring to $\mathcal{X}$ at $x_0$ exists. Given a pair $A$, $A'$ of these, then $A \cong A'[[t_1, \ldots , t_ r]]$ or $A' \cong A[[t_1, \ldots , t_ r]]$ as $S$-algebras for some $r$.

Proof. By Lemma 105.2.3 and Formal Deformation Theory, Lemma 88.13.4 (note that the assumptions of this lemma hold by Formal Deformation Theory, Lemmas 88.16.6 and Definition 88.16.8). By the uniquness result of Formal Deformation Theory, Lemma 88.14.5 there exists a “minimal” versal ring $A$ of $\mathcal{X}$ at $x_0$ such that any other versal ring of $\mathcal{X}$ at $x_0$ is isomorphic to $A[[t_1, \ldots , t_ r]]$ for some $r$. This clearly implies the second statement. $\square$


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