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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.

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    It would be helpful to know: a) WHICH proof you mean and b) WHICH steps in this proof are not clear to you.2012-11-20
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    Is $X$ an integral variety ?2012-11-20
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    I asked a professor, who answered me "go to affine components, pass to the quotient, take the intersection", and I felt at lost. $X$ is a projective variety in $\mathbb{P}^{n}$.2012-11-21
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    @user32240: In this case (if I am right about what a projective variety is in your context), a proof of this fact can be found in Hartshorne "Algebraic Geometry", Ch.1, Theorem 3.4. Does this help?2012-11-21
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    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

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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.

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    Your proof did not use $X$ being projective in any essential way. I have some doubt if it works.2012-11-21
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    The proof is 2 parts. The first (which I only cite) is that $\mathbb{P}^n$ is proper as a variety, and hence so is any closed subvariety namely $X$ (this is where we use $X$ projective). The second is that for any irreducible proper variety $Y$, $\Gamma(O_Y, Y) = k$.2012-11-21
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    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