next up previous
Next: A convention. Up: Algebraic geometry and Ring Previous: Algebraic geometry and Ring

sheaves

Affine spectrum $ \operatorname{Spec}(A)$ of a ring $ A$ carries one more important structure. Namely, its structure sheaf.

We will firstly review some definitions and first properties of sheaves.

To illustrate the idea, we recall an easy lemma in topology.

LEMMA 06.1 (Gluing lemma)   Let $ X,Y$ be a topological spaces. Let $ \{U_\lambda\}_{\lambda \in \Lambda}$ be an open covering of $ X$ .

  1. If we are given a collection of continuous maps $ \{f_\lambda: U_\lambda \to Y\}_{\lambda\in \Lambda}$ such that

    $\displaystyle f_\lambda\vert _{U_{\lambda}\cap U_{\mu}}
=
f_\mu\vert _{U_{\lambda}\cap U_{\mu}}
$

    holds for any pair $ (\lambda,\mu)\in \Lambda^2$ , then we have a unique continuous map $ f:X\to Y$ such that

    $\displaystyle f\vert _{U_\lambda}=f_\lambda
$

    holds for any $ \lambda \in \Lambda$ .

  2. Conversely, if we are given a continuous map $ f:X\to Y$ , then we obtain a collection of maps $ \{f_\lambda: U_\lambda \to Y\}_{\lambda\in \Lambda}$ by restriction.

PROOF.. (1) It is easy to verify that we have a well-defined map

$\displaystyle f: X\to Y
$

with

$\displaystyle f\vert _{U_\lambda}=f_\lambda.
$

The continuity of $ f$ is proved by verifying that the inverse image of any open set $ V\subset Y$ by $ f$ is open in $ X$ . % latex2html id marker 1784
$ \qedsymbol$



Subsections
next up previous
Next: A convention. Up: Algebraic geometry and Ring Previous: Algebraic geometry and Ring
2017-07-21