# Definition:Radical of Ideal of Ring

## Definition

Let $A$ be a commutative ring with unity.

Let $I$ be an ideal of $A$.

### Definition 1

The radical of $I$ is the ideal of elements of which some power is in $I$:

$\map \Rad I := \set {a \in A: \exists n \in \N : a^n \in I}$

### Definition 2

Let $A / I$ be the quotient ring.

Let $\Nil {A / I}$ be its nilradical.

Let $\pi: A \to A / I$ be the quotient mapping.

The radical of $I$ is the preimage of $\Nil {A / I}$ under $\pi$:

$\map \Rad I = \pi^{-1} \sqbrk {\Nil {A / I} }$

## Also denoted as

The radical of $I$ is also denoted as $\sqrt I$ or $\map r I$.

The notation $\sqrt I$ is not recommended, as it is to easy to conflate with the sign for the square root, and so will not be used on $\mathsf{Pr} \infty \mathsf{fWiki}$.