Lemma 53.22.2 (0E63)—The Stacks project

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Table of contents
Part 3: Topics in Scheme Theory
Chapter 53: Algebraic Curves
Section 53.22: Contracting rational tails
Lemma 53.22.2 (cite)

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Lemma 53.22.2. Let $k$ be a field. Let $X$ be a proper scheme over $k$ having dimension $1$ and $H^0(X, \mathcal{O}_ X) = k$ . Assume the singularities of $X$ are at-worst-nodal. Let $C \subset X$ be a rational tail (Example 53.22.1). Then $\deg (\omega _ X|_ C) < 0$ .

Proof. Let $X' \subset X$ be as in the example. Then we have a short exact sequence

$0 \to \omega _ C \to \omega _ X|_ C \to \mathcal{O}_{C \cap X'} \to 0$

See Lemmas 53.4.6, 53.19.16, and 53.19.17. With $k'$ as in the example we see that $\deg (\omega _ C) = -2[k' : k]$ as $C \cong \mathbf{P}^1_{k'}$ by Proposition 53.10.4 and $\deg (C \cap X') = [k' : k]$ . Hence $\deg (\omega _ X|_ C) = -[k' : k]$ which is negative. $\square$

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