Lemma 90.13.3 (06SZ)—The Stacks project

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Table of contents
Part 6: Deformation Theory
Chapter 90: Formal Deformation Theory
Section 90.13: Versal formal objects
Lemma 90.13.3 (cite)

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Lemma 90.13.3. Let $\mathcal{F}$ be a category cofibred in groupoids over $\mathcal{C}_\Lambda$ which has (S1). Let $B \to A$ be a surjection in $\mathcal{C}_\Lambda$ with kernel $I$ annihilated by $\mathfrak m_ B$ . Let $x \in \mathcal{F}(A)$ . The set of ideals

$\mathcal{J} = \{ J \subset I \mid \text{there exists an }y \to x\text{ lying over }B/J \to A\}$

has a smallest element.

Proof. Note that $\mathcal{J}$ is nonempty as $I \in \mathcal{J}$ . Also, if $J \in \mathcal{J}$ and $J \subset J' \subset I$ then $J' \in \mathcal{J}$ because we can pushforward the object $y$ to an object $y'$ over $B/J'$ . Let $J$ and $K$ be elements of the displayed set. We claim that $J \cap K \in \mathcal{J}$ which will prove the lemma. Since $I$ is a $k$ -vector space we can find an ideal $J \subset J' \subset I$ such that $J \cap K = J' \cap K$ and such that $J' + K = I$ . By the above we may replace $J$ by $J'$ and assume that $J + K = I$ . In this case

$A/(J \cap K) = A/J \times _{A/I} A/K.$

Hence the existence of an element $z \in \mathcal{F}(A/(J \cap K))$ mapping to $x$ follows, via (S1), from the existence of the elements we have assumed exist over $A/J$ and $A/K$ . $\square$

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