Lemma 90.26.1. Let \mathcal{F} be a category cofibered in groupoids over \mathcal{C}_\Lambda . Let U : \mathcal{C}_\Lambda \to \textit{Sets} be a functor. Let f : U \to \mathcal{F} be a smooth morphism of categories cofibered in groupoids. Then:
90.26 Presentations of deformation categories
According to the next lemma, a smooth morphism from a predeformation functor to a predeformation category \mathcal{F} gives rise to a presentation of \mathcal{F} by a smooth groupoid in functors.
Proof. From the construction of Lemma 90.25.2 we have a commutative diagram
\xymatrix{ R = U \times _{f, \mathcal{F}, f} U \ar[r]_-s \ar[d]_ t & U \ar[d]^ f \\ U \ar[r]^ f & \mathcal{F} }
where t, s are the first and second projections. So t, s are smooth by Lemma 90.8.7. Hence (1) holds.
If the assumption of (2) holds, then by Lemma 90.8.8 the morphism f : U \to \mathcal{F} is essentially surjective. Hence by Lemma 90.25.3 the morphism [f] : [U/R] \to \mathcal{F} is an equivalence. \square
Lemma 90.26.2. Let \mathcal{F} be a deformation category. Let U : \mathcal{C}_\Lambda \to \textit{Sets} be a deformation functor. Let f: U \to \mathcal{F} be a morphism of categories cofibered in groupoids. Then U \times _{f, \mathcal{F}, f} U is a deformation functor with tangent space fitting into an exact sequence of k-vector spaces
0 \to \text{Inf}(\mathcal{F}) \to T(U \times _{f, \mathcal{F}, f} U) \to TU \oplus TU
Proof. Follows from Lemma 90.20.1 and the fact that \text{Inf}(U) = (0). \square
Lemma 90.26.3. Let \mathcal{F} be a deformation category. Let U : \mathcal{C}_\Lambda \to \textit{Sets} be a prorepresentable functor. Let f : U \to \mathcal{F} be a morphism of categories cofibered in groupoids. Let (U, R, s, t, c) be the groupoid in functors on \mathcal{C}_\Lambda constructed from f : U \to \mathcal{F} in Lemma 90.25.2. If \dim _ k \text{Inf}(\mathcal{F}) < \infty , then (U, R, s, t, c) is prorepresentable.
Proof. Note that U is a deformation functor by Example 90.16.10. By Lemma 90.26.2 we see that R = U \times _{f, \mathcal{F}, f} U is a deformation functor whose tangent space TR = T(U \times _{f, \mathcal{F}, f} U) sits in an exact sequence 0 \to \text{Inf}(\mathcal{F}) \to TR \to TU \oplus TU. Since we have assumed the first space has finite dimension and since TU has finite dimension by Example 90.11.11 we see that \dim TR < \infty . The map \gamma : \text{Der}_\Lambda (k, k) \to TR see (90.12.6.1) is injective because its composition with TR \to TU is injective by Theorem 90.18.2 for the prorepresentable functor U. Thus R is prorepresentable by Theorem 90.18.2. It follows from Lemma 90.22.4 that (U, R, s, t, c) is prorepresentable. \square
Theorem 90.26.4. Let \mathcal{F} be a category cofibered in groupoids over \mathcal{C}_\Lambda . Then \mathcal{F} admits a presentation by a smooth prorepresentable groupoid in functors on \mathcal{C}_\Lambda if and only if the following conditions hold:
\mathcal{F} is a deformation category.
\dim _ k T\mathcal{F} is finite.
\dim _ k \text{Inf}(\mathcal{F}) is finite.
Proof. Recall that a prorepresentable functor is a deformation functor, see Example 90.16.10. Thus if \mathcal{F} is equivalent to a smooth prorepresentable groupoid in functors, then conditions (1), (2), and (3) follow from Lemma 90.24.2 (1), (2), and (3).
Conversely, assume conditions (1), (2), and (3) hold. Condition (1) implies that (S1) and (S2) are satisfied, see Lemma 90.16.6. By Lemma 90.13.4 there exists a versal formal object \xi . Setting U = \underline{R}|_{\mathcal{C}_\Lambda } the associated map \underline{\xi } : U \to \mathcal{F} is smooth (this is the definition of a versal formal object). Let (U, R, s, t, c) be the groupoid in functors constructed in Lemma 90.25.2 from the map \underline{\xi }. By Lemma 90.26.1 we see that (U, R, s, t, c) is a smooth groupoid in functors and that [U/R] \to \mathcal{F} is an equivalence. By Lemma 90.26.3 we see that (U, R, s, t, c) is prorepresentable. Hence [U/R] \to \mathcal{F} is the desired presentation of \mathcal{F}. \square
Comments (0)