# Homogenous localization and normal localization in graded modules

Let $R$ be a graded ring. There are two ways to take the localization of $R$.

Let $\mathfrak{p}$ be a homogenous prime ideal, $T$ be the set of all homogenous element of $S\setminus \mathfrak{p}$. Then $R_{(\mathfrak{p})}$ the subring of $T^{-1}R$ consisting of all $\dfrac{f}{g}$ where $f$ and $g$ are homogenous of the same degree is called homogenous localization of $R$.

Let $R$ be a graded ring, $S\subset R$ is a multiplicative closed subset of $R$. For any $f\in R, g\in S$ define the degree of $\dfrac{f}{g}$ to be deg$f$-deg$g$. Then it is not hard to check that this is well defined. The localization $S^{-1}R$ is a graded ring.

My question are the follows:

• What is the difference between these two localizations?

• What are the applications of them in higher commutative algebra ?

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I think in the second case, $S$ must contain only homogeneous elements and one defines the degree of $f/g$ only when $f$ is homogeneous. Otherwise you don't get a graded ring $S^{-1}R$ by you construction. – QiL'8 May 20 '12 at 6:46
@Qil :No, I do not think so. We can think $\dfrac{f}{g}$ as a polynomial of degree degf-deg g. – Arsenaler May 20 '12 at 8:06
Can you show that the set of elements of a given degree (including the element $0$) is stable by addition ? – QiL'8 May 20 '12 at 10:26

An arbitrary element of $R$ has no degree. Only the homogeneous elements have some degree. You seem to be confused with the notion of degree of polynomials. But here, there are no polynomials at all.
If $S \subseteq R$ is a homogeneous submonoid, we can grade the usual localization $S^{-1} R=R_S$ (i.e. of the underlying rings) as follows: The homogeneous elements of degree $d$ are those of the form $r/s$, where $r \in R$, $s \in S$ are homogeneous elements such that the degree of $r$ equals $d$ plus the degree of $s$. As always the homogeneous elements of degree $0$ constitute a subring. This is usually called $R_{(S)}$. When $\mathfrak{p}$ is some homogeneous prime ideal, then the set of homogeneous elements $s \in R$ such that $s \notin \mathfrak{p}$ is an example for $S$. Therefoe, we have a graded ring $R_{\mathfrak{p}}$ and the subring of elements of degree $0$ is called $R_{(\mathfrak{p})}$.