4
$\begingroup$

A commutative ring is called zero-dimensional if all its prime ideals are maximal, and a ring is said to have a bounded index of nilpotency if there is a positive integer $n$ such that $x^n = 0$ for every nilpotent $x$ in the ring. Does every zero-dimensional commutative ring have a bounded index of nilpotency?

2 Answers 2

2

What ahout $k[x_2,x_3,\dots]/(x_2^2,x_3^3,x_4^4,\dots)$?

  • 0
    A very nice example again, Mariano!2012-10-27
1

Here is a kind of explanation and generalization of Mariano's example.

It is known that a ring $R$ is zero-dimensional if and only if for all $x \in R$ there is some $n \geq 1$ such that $x^{n+1} | x^n$. It follows that zero-dimensional rings are closed unter directed colimits. Alternatively, this follows from the fact that $\mathrm{Spec} : \mathsf{Ring} \to \mathsf{Top}^{\mathrm{op}}$ preserves directed colimits.

In particular it is quite easy to construct arbitrary "large" zero-dimensional rings! But of course directed colimits can destroy bounded index of nilpotency.