Let $R$ be a noetherian ring, and let $I$ be a proper ideal in $R$. If $I$ is generated by $n$ elements, we have by Krull's Principal Ideal Theorem that the height of $I$ is at most $n$. Is it true that $\operatorname{dim}R/I \ge \operatorname{dim}R - n$?
Coheight of an ideal generated by $n$ elements
3
$\begingroup$
commutative-algebra
-
0Edited to make question clearer to future viewers – 2012-05-22
1 Answers
4
The answer is no. Let $R=\mathbb{Z}_{(2)}[X]$, and $I=(2X-1)$, where $\mathbb{Z}_{(2)}$ is the ring $\mathbb{Z}$ localizing at the prime ideal $(2)$. Then $R/I$ is a field.
Another cheap counter-example is $R=k\times \mathbb{Z}[X]$ where $k$ is any field. Let $I$ generated by $(0,1)$, i.e., $I=\{0\}\times \mathbb{Z}[X]$. Then $R/I$ is a field. However $\dim R=2$.
It is true for Noetherian local ring $(R,\mathfrak{m})$. Suppose $\dim R/I=r$. Then we can find $x_1,\ldots,x_r$ whose image generating an ideal of definition of $R/I$, that is, $\mathfrak{m}/I$ is minimal over $(x_1,\ldots,x_r)/I$. Say $y_1,\ldots,y_n$ generate $I$, then $\mathfrak{m}$ is minimal over $(x_1,\ldots,x_r,y_1,\ldots,y_n)$, so $\dim R\leq r+n=\dim R/I+n$.
-
0If you know that every maximal ideal has height 3 in your ring, the problem is solved. – 2012-05-22