The Stacks project

93.16 Unobstructed deformation problems

Let $p : \mathcal{F} \to \mathcal{C}_\Lambda $ be a category cofibred in groupoids. Recall that we say $\mathcal{F}$ is smooth or unobstructed if $p$ is smooth. This means that given a surjection $\varphi : A' \to A$ in $\mathcal{C}_\Lambda $ and $x \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{F}(A))$ there exists a morphism $f : x' \to x$ in $\mathcal{F}$ with $p(f) = \varphi $. See Formal Deformation Theory, Section 90.9. In this section we give some geometrically meaningful examples.

Lemma 93.16.1. In Example 93.8.1 let $P$ be a local complete intersection over $k$ (Algebra, Definition 10.135.1). Then $\mathcal{D}\! \mathit{ef}_ P$ is unobstructed.

Proof. Let $(A, Q) \to (k, P)$ be an object of $\mathcal{D}\! \mathit{ef}_ P$. Then we see that $A \to Q$ is a syntomic ring map by Algebra, Definition 10.136.1. Hence for any surjection $A' \to A$ in $\mathcal{C}_\Lambda $ we see that there is a morphism $(A', Q') \to (A, Q)$ lifting $A' \to A$ by Smoothing Ring Maps, Proposition 16.3.2. This proves the lemma. $\square$

Lemma 93.16.2. In Situation 93.9.9 if $U_{12} \to \mathop{\mathrm{Spec}}(k)$ is smooth, then the morphism

\[ \mathcal{D}\! \mathit{ef}_ X \longrightarrow \mathcal{D}\! \mathit{ef}_{U_1} \times \mathcal{D}\! \mathit{ef}_{U_2} = \mathcal{D}\! \mathit{ef}_{P_1} \times \mathcal{D}\! \mathit{ef}_{P_2} \]

is smooth. If in addition $U_1$ is a local complete intersection over $k$, then

\[ \mathcal{D}\! \mathit{ef}_ X \longrightarrow \mathcal{D}\! \mathit{ef}_{U_2} = \mathcal{D}\! \mathit{ef}_{P_2} \]

is smooth.

Proof. The equality signs hold by Lemma 93.9.7. Let us think of $\mathcal{C}_\Lambda $ as a deformation category over $\mathcal{C}_\Lambda $ as in Formal Deformation Theory, Section 90.9. Then

\[ \mathcal{D}\! \mathit{ef}_{P_1} \times \mathcal{D}\! \mathit{ef}_{P_2} = \mathcal{D}\! \mathit{ef}_{P_1} \times _{\mathcal{C}_\Lambda } \mathcal{D}\! \mathit{ef}_{P_2}, \]

see Formal Deformation Theory, Remarks 90.5.2 (14). Using Lemma 93.9.10 the first statement is that the functor

\[ \mathcal{D}\! \mathit{ef}_{P_1} \times _{\mathcal{D}\! \mathit{ef}_{P_{12}}} \mathcal{D}\! \mathit{ef}_{P_2} \longrightarrow \mathcal{D}\! \mathit{ef}_{P_1} \times _{\mathcal{C}_\Lambda } \mathcal{D}\! \mathit{ef}_{P_2} \]

is smooth. This follows from Formal Deformation Theory, Lemma 90.20.2 as long as we can show that $T\mathcal{D}\! \mathit{ef}_{P_{12}} = (0)$. This vanishing follows from Lemma 93.8.4 as $P_{12}$ is smooth over $k$. For the second statement it suffices to show that $\mathcal{D}\! \mathit{ef}_{P_1} \to \mathcal{C}_\Lambda $ is smooth, see Formal Deformation Theory, Lemma 90.8.7. In other words, we have to show $\mathcal{D}\! \mathit{ef}_{P_1}$ is unobstructed, which is Lemma 93.16.1. $\square$

Lemma 93.16.3. In Example 93.9.1 let $X$ be a scheme over $k$. Assume

  1. $X$ is separated, finite type over $k$ and $\dim (X) \leq 1$,

  2. $X \to \mathop{\mathrm{Spec}}(k)$ is smooth except at the closed points $p_1, \ldots , p_ n \in X$.

Let $\mathcal{O}_{X, p_1}$, $\mathcal{O}_{X, p_1}^ h$, $\mathcal{O}_{X, p_1}^\wedge $ be the local ring, henselization, completion. Consider the maps of deformation categories

\[ \mathcal{D}\! \mathit{ef}_ X \longrightarrow \prod \mathcal{D}\! \mathit{ef}_{\mathcal{O}_{X, p_ i}} \longrightarrow \prod \mathcal{D}\! \mathit{ef}_{\mathcal{O}_{X, p_ i}^ h} \longrightarrow \prod \mathcal{D}\! \mathit{ef}_{\mathcal{O}_{X, p_ i}^\wedge } \]

The first arrow is smooth and the second and third arrows are smooth and induce isomorphisms on tangent spaces.

Proof. Choose an affine open $U_2 \subset X$ containing $p_1, \ldots , p_ n$ and the generic point of every irreducible component of $X$. This is possible by Varieties, Lemma 33.43.3 and Properties, Lemma 28.29.5. Then $X \setminus U_2$ is finite and we can choose an affine open $U_1 \subset X \setminus \{ p_1, \ldots , p_ n\} $ such that $X = U_1 \cup U_2$. Set $U_{12} = U_1 \cap U_2$. Then $U_1$ and $U_{12}$ are smooth affine schemes over $k$. We conclude that

\[ \mathcal{D}\! \mathit{ef}_ X \longrightarrow \mathcal{D}\! \mathit{ef}_{U_2} \]

is smooth by Lemma 93.16.2. Applying Lemmas 93.9.7 and 93.15.1 we win. $\square$

Lemma 93.16.4. In Example 93.9.1 let $X$ be a scheme over $k$. Assume

  1. $X$ is separated, finite type over $k$ and $\dim (X) \leq 1$,

  2. $X$ is a local complete intersection over $k$, and

  3. $X \to \mathop{\mathrm{Spec}}(k)$ is smooth except at finitely many points.

Then $\mathcal{D}\! \mathit{ef}_ X$ is unobstructed.

Proof. Let $p_1, \ldots , p_ n \in X$ be the points where $X \to \mathop{\mathrm{Spec}}(k)$ isn't smooth. Choose an affine open $U_2 \subset X$ containing $p_1, \ldots , p_ n$ and the generic point of every irreducible component of $X$. This is possible by Varieties, Lemma 33.43.3 and Properties, Lemma 28.29.5. Then $X \setminus U_2$ is finite and we can choose an affine open $U_1 \subset X \setminus \{ p_1, \ldots , p_ n\} $ such that $X = U_1 \cup U_2$. Set $U_{12} = U_1 \cap U_2$. Then $U_1$ and $U_{12}$ are smooth affine schemes over $k$. We conclude that

\[ \mathcal{D}\! \mathit{ef}_ X \longrightarrow \mathcal{D}\! \mathit{ef}_{U_2} \]

is smooth by Lemma 93.16.2. Applying Lemmas 93.9.7 and 93.16.1 we win. $\square$


Comments (0)


Post a comment

Your email address will not be published. Required fields are marked.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).

Unfortunately JavaScript is disabled in your browser, so the comment preview function will not work.

All contributions are licensed under the GNU Free Documentation License.




In order to prevent bots from posting comments, we would like you to prove that you are human. You can do this by filling in the name of the current tag in the following input field. As a reminder, this is tag 0DZG. Beware of the difference between the letter 'O' and the digit '0'.