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From Hartshorne, Chapter II.2, Proposition 2.5(b).

If $R$ is a graded ring and $\mathfrak a$ is a homogenous ideal, then the function defined as

$$\phi(\mathfrak a) = \mathfrak aR_f\cap R_{(f)}$$

is a bijection, where $R_f$ is the usual localization of $R$ with respect to the multiplicative set $S=\{1,f,f²,...\}$ and $R_{(f)}$ is the subring of homogeneous elements of degree $0$ of $R_f$.

user26857
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  • If I'm not mistaken the quoted result from Hartshorne looks different from what you asked here, and the same thing can be said about the accepted answer which actually deals with the result from the book. – user26857 Oct 25 '14 at 08:51

1 Answers1

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In EGA II, Proposition 2.3.6, there is a nice proof of the fact that the map $\psi_f: D_+(f)\rightarrow\operatorname{Spec}(S_{(f)})$ given by $\mathfrak{p}\rightarrow \mathfrak{p}_f\cap S_{(f)}$ is a homeomorphism.
Let $f\in S_d$. The map $\psi_f$ is continuous because for $g\in S_e$ you have $g^d/f^e\notin\psi_f(\mathfrak{p})=\{x/f^m:x\in \mathfrak{p}_{md}\}$ if and only if $g\notin\mathfrak{p}$, and the sets $D(g^d/f^e)$ for $g\in S_e$, $e>0$, form a basis for $\operatorname{Spec}(S_{(f)})$ (for any $x/f^m\in S_{(f)}$, $D(x/f^m)=D(x^d/f^{md})=D(x^{\deg(f)}/f^{\deg(x)})$). Once we proved that $\psi_f$ is injective, then this will imply that $\psi_f$ is a homeomorphism of $D_+(f)$ with its image (the sets $D_+(fg)$ form a basis for the topology of $D_+(f)$).

To show that $\psi_f$ is injective, since $D_+(f)$ is $T_0$ (it has the induced topology of $\operatorname{Spec}(S)$), given two elements in $D_+(f)$ there exists $g$ , say of degree $e$, such that $D_+(fg)$ contains one of them but not the other. According to what we have proved above, just the image of one of them is contained in $D(g^d/f^e)$.

Finally, to show that $\psi_f$ is surjective, given $\mathfrak{q}_0\in\operatorname{Spec}(S_{(f)})$, let $\mathfrak{p}_n=\{x\in S_n:x^d/f^n\in\mathfrak{q}_0\}$ and check that $\mathfrak{p}=\oplus\mathfrak{p}_n$ is a prime ideal in $D_+(f)$ such that $\psi(\mathfrak{p})=\mathfrak{q}_0$.

user26857
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Diego
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