The Stacks project

Lemma 33.33.2. Let $k$ be a field. Let $X$ be a proper scheme over $k$. Let $0 \to \mathcal{F}_1 \to \mathcal{F}_2 \to \mathcal{F}_3 \to 0$ be a short exact sequence of coherent modules on $X$. Then

\[ \chi (X, \mathcal{F}_2) = \chi (X, \mathcal{F}_1) + \chi (X, \mathcal{F}_3) \]

Proof. Consider the long exact sequence of cohomology

\[ 0 \to H^0(X, \mathcal{F}_1) \to H^0(X, \mathcal{F}_2) \to H^0(X, \mathcal{F}_3) \to H^1(X, \mathcal{F}_1) \to \ldots \]

associated to the short exact sequence of the lemma. The rank-nullity theorem in linear algebra shows that

\[ 0 = \dim H^0(X, \mathcal{F}_1) - \dim H^0(X, \mathcal{F}_2) + \dim H^0(X, \mathcal{F}_3) - \dim H^1(X, \mathcal{F}_1) + \ldots \]

This immediately implies the lemma. $\square$

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