1
$\begingroup$

Assume we have a projective variety $X$ over some algebraically closed field $k$. How can we show that $O_{X}=k$? I tried to do it in simple examples but the proof is not clear to me.

  • 0
    This does help. The professor said I can follow Hartshorne, but I thought that must be over my head and did not really try that.2012-11-21

1 Answers 1

3

Maybe I'm being dumb, but I think you want $X$ to be irreducible, otherwise consider say $X = $2 points sitting around in $\mathbb{P}^1$, $\Gamma$ is $k \oplus k$.

I'm also working with $X$ a variety, i.e. $O_X$ nilpotent free, I hope that's cool.

For $X$ irreducible, a proof follows from knowing that $\mathbb{P}^n$ is proper over $k$, hence so is any closed subvariety, in particular $X$. With this, a global section is the same data as a regular map $X \rightarrow \mathbb{A}^1$, the image must be closed by properess, hence finitely many points, hence only one point by irreducibility.

  • 1
    You can replace irreducible with connected (assuming $X$ is reduced). In the proof you should say why the image of $X\to \mathbb A^1$ is the whole space.2012-11-22