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.
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.
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 is smooth. If in addition $U_1$ is a local complete intersection over $k$, then 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
see Formal Deformation Theory, Remarks 90.5.2 (14). Using Lemma 93.9.10 the first statement is that the functor
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
$X$ is separated, finite type over $k$ and $\dim (X) \leq 1$,
$X \to \mathop{\mathrm{Spec}}(k)$ 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
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
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
$X$ is separated, finite type over $k$ and $\dim (X) \leq 1$,
$X$ is a local complete intersection over $k$, and
$X \to \mathop{\mathrm{Spec}}(k)$ 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
is smooth by Lemma 93.16.2. Applying Lemmas 93.9.7 and 93.16.1 we win. $\square$
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