The Stacks project

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. $\square$