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I would like to know a reference of the following statement (or counter example).

Let $S$ be a (commutative) Noetherian standard graded ring over a local ring, i.e., $S = S_0[S_1]$, where $S_0$ is Noetherian local and $S_1$ is finitely generated over $S_0$. If $S$ is an integral domain, then $X = \operatorname{Proj} S$ is a irreducible and reduced. Hence $\mathcal{O}_{X,x}$ is an integral domain for any $x \in X$. But what about equidimensionality? That is

Let $S$ be a Noetherian standard graded equidimensional ring over a Noetherian local ring. Then is the local ring $\mathcal{O}_{X,x}$ equidimensional for all $x \in X$?

A ring $R$ is called $\textit{equidimensional}$ if $\dim R = \dim R/p$ for any minimal prime $p$ of $R$.

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  • $\begingroup$ No, it is a stalk on a Proj not Spec. Also, $x$ is any point not necessarily closed. $\endgroup$ – Youngsu Dec 2 '14 at 19:48
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Here is a counter example. Set $S_0 = k[x]$ where $k$ is a field. Let $S = S_0[Y, Z, W]/(xY, xZ)$ with $Y$, $Z$, $W$ in degree $1$. Then $X = \text{Proj}(S)$ is the union of a copy of $\text{Spec}(S_0) \cong \mathbf{A}^1_k$ and a copy of $\mathbf{P}^2_k$ glued in a point $x$. Hence $\mathcal{O}_{X, x}$ is not equidimensional.

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    $\begingroup$ I do not think $k[x][Y,Z,W]/(xY,xZ)$ is equidimensional (as a ring). The ideal $(xY,xZ)$ is equal to $x(Y,Z) = (x) \cap (Y,Z)$ in $k[x][Y,Z,W]$, and $\dim k[x][Y,Z,W]/(x) = 3$, but $\dim k[x][Y,Z,W]/ (Y,Z) =2$. $\endgroup$ – Youngsu Jun 12 '15 at 18:49

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