Contents

### Context

#### Algebra

higher algebra

universal algebra

# Contents

## Definition

Given a unital (typically noncommutative) ring $R$, the Jacobson radical $J(R)$ is defined as the set of elements $r\in R$ satisfying the following equivalent properties:

1. For each simple left $R$-module $M$, $r M =0$.
2. Each maximal left ideal of $R$ contains $r$.
3. For all $x\in R$, $1 - r x$ is left invertible in $R$.

Alternatively,

1. $J(R)$ is the intersection of all maximal left ideals of $R$.
2. $J(R)$ is the intersection of all maximal right ideals of $R$.

The properties required remain the same if one interchanges left and right (modules, invertibility etc.) i.e. $J(R)=J(R^{op})$.

$J(R)$ is a $2$-sided ideal in $R$. The rings for which $J(R)=0$ are called semiprimitive rings. In other words, for each nonzero element $r$ in a semiprimitive ring, by the definition, there is a simple module not left annihilated by $r$. Given any ring $R$, the quotient $R/J(R)$ is semiprimitive.

## Examples

###### Example

(Jacobson radical of formal power series algebra)

The Jacobson radical of a formal power series algebra consists of those formal power series whose constant term vanishes.