Lemma 33.16.4. Let $f : X \to S$ be a morphism of schemes. Let $x \in X$. There is a canonical isomorphism

$T_{X/S, x} = \mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_{X, x}}(\Omega _{X/S, x}, \kappa (x))$

of vector spaces over $\kappa (x)$.

Proof. Set $\kappa = \kappa (x)$. Given $\theta \in T_{X/S, x}$ we obtain a map

$\theta ^*\Omega _{X/S} \to \Omega _{\mathop{\mathrm{Spec}}(\kappa [\epsilon ])/\mathop{\mathrm{Spec}}(\kappa (s))} \to \Omega _{\mathop{\mathrm{Spec}}(\kappa [\epsilon ])/\mathop{\mathrm{Spec}}(\kappa )}$

Taking sections we obtain an $\mathcal{O}_{X, x}$-linear map $\xi _\theta : \Omega _{X/S, x} \to \kappa \text{d}\epsilon$, i.e., an element of the right hand side of the formula of the lemma. To show that $\theta \mapsto \xi _\theta$ is an isomorphism we can replace $S$ by $s$ and $X$ by the scheme theoretic fibre $X_ s$. Indeed, both sides of the formula only depend on the scheme theoretic fibre; this is clear for $T_{X/S, x}$ and for the RHS see Morphisms, Lemma 29.32.10. We may also replace $X$ by the spectrum of $\mathcal{O}_{X, x}$ as this does not change $T_{X/S, x}$ (Schemes, Lemma 26.13.1) nor $\Omega _{X/S, x}$ (Modules, Lemma 17.27.7).

Let $(A, \mathfrak m, \kappa )$ be a local ring over a field $k$. To finish the proof we have to show that any $A$-linear map $\xi : \Omega _{A/k} \to \kappa$ comes from a unique $k$-algebra map $\varphi : A \to \kappa [\epsilon ]$ agreeing with the canonical map $c : A \to \kappa$ modulo $\epsilon$. Write $\varphi (a) = c(a) + D(a) \epsilon$ the reader sees that $a \mapsto D(a)$ is a $k$-derivation. Using the universal property of $\Omega _{A/k}$ we see that each $D$ corresponds to a unique $\xi$ and vice versa. This finishes the proof. $\square$

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