Loading [MathJax]/extensions/tex2jax.js

The Stacks project

Lemma 29.31.2. Let $i : Z \to X$ be an immersion. The conormal sheaf of $i$ has the following properties:

  1. Let $U \subset X$ be any open subscheme such that $i$ factors as $Z \xrightarrow {i'} U \to X$ where $i'$ is a closed immersion. Let $\mathcal{I} = \mathop{\mathrm{Ker}}((i')^\sharp ) \subset \mathcal{O}_ U$. Then

    \[ \mathcal{C}_{Z/X} = (i')^*\mathcal{I}\quad \text{and}\quad i'_*\mathcal{C}_{Z/X} = \mathcal{I}/\mathcal{I}^2 \]
  2. For any affine open $\mathop{\mathrm{Spec}}(R) = U \subset X$ such that $Z \cap U = \mathop{\mathrm{Spec}}(R/I)$ there is a canonical isomorphism $\Gamma (Z \cap U, \mathcal{C}_{Z/X}) = I/I^2$.

Proof. Mostly clear from the definitions. Note that given a ring $R$ and an ideal $I$ of $R$ we have $I/I^2 = I \otimes _ R R/I$. Details omitted. $\square$


Comments (0)


Your email address will not be published. Required fields are marked.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).

Unfortunately JavaScript is disabled in your browser, so the comment preview function will not work.

All contributions are licensed under the GNU Free Documentation License.