Lemma 93.10.3. In Example 93.9.1 let $f : X \to Y$ be a morphism of schemes over $k$. There is a canonical exact sequence of $k$-vector spaces
Proof. The obvious map of deformation categories $\mathcal{D}\! \mathit{ef}_{X \to Y} \to \mathcal{D}\! \mathit{ef}_ X \times \mathcal{D}\! \mathit{ef}_ Y$ gives two of the arrows in the exact sequence of the lemma. Recall that $\text{Inf}(\mathcal{D}\! \mathit{ef}_{X \to Y})$ is the set of automorphisms of the trivial deformation
of $X \to Y$ to $k[\epsilon ]$ equal to the identity modulo $\epsilon $. This is clearly the same thing as pairs $(\alpha , \beta ) \in \text{Inf}(\mathcal{D}\! \mathit{ef}_ X \times \mathcal{D}\! \mathit{ef}_ Y)$ of infinitesimal automorphisms of $X$ and $Y$ compatible with $f'$, i.e., such that $f' \circ \alpha = \beta \circ f'$. By Deformation Theory, Lemma 91.7.1 for an arbitrary pair $(\alpha , \beta )$ the difference between the morphism $f' : X' \to Y'$ and the morphism $\beta ^{-1} \circ f' \circ \alpha : X' \to Y'$ defines an element in
Equality by More on Morphisms, Lemma 37.13.3. This defines the last top horizontal arrow and shows exactness in the first two places. For the map
we interpret elements of the source as morphisms $f_\epsilon : X' \to Y'$ over $\mathop{\mathrm{Spec}}(k[\epsilon ])$ equal to $f$ modulo $\epsilon $ using Deformation Theory, Lemma 91.7.1. We send $f_\epsilon $ to the isomorphism class of $(f_\epsilon : X' \to Y')$ in $T\mathcal{D}\! \mathit{ef}_{X \to Y}$. Note that $(f_\epsilon : X' \to Y')$ is isomorphic to the trivial deformation $(f' : X' \to Y')$ exactly when $f_\epsilon = \beta ^{-1} \circ f \circ \alpha $ for some pair $(\alpha , \beta )$ which implies exactness in the third spot. Clearly, if some first order deformation $(f_\epsilon : X_\epsilon \to Y_\epsilon )$ maps to zero in $T(\mathcal{D}\! \mathit{ef}_ X \times \mathcal{D}\! \mathit{ef}_ Y)$, then we can choose isomorphisms $X' \to X_\epsilon $ and $Y' \to Y_\epsilon $ and we conclude we are in the image of the south-west arrow. Therefore we have exactness at the fourth spot. Finally, given two first order deformations $X_\epsilon $, $Y_\epsilon $ of $X$, $Y$ there is an obstruction in
which vanishes if and only if $f : X \to Y$ lifts to $X_\epsilon \to Y_\epsilon $, see Deformation Theory, Lemma 91.7.1. This finishes the proof. $\square$
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