# Ore extension

In mathematics, especially in the area of algebra known as ring theory, an Ore extension, named after Øystein Ore, is a special type of a ring extension whose properties are relatively well understood. Ore extensions appear in several natural contexts, including skew and differential polynomial rings, group algebras of polycyclic groups, universal enveloping algebras of solvable Lie algebras, and coordinate rings of quantum groups.

## Definition

Suppose that R is a ring, σ:RR is an injective ring homomorphism, and δ:RR is a σ-derivation of R, which means that δ is a homomorphism of abelian groups satisfying

$\delta(r_1 r_2)=(\sigma r_1)\delta r_2+(\delta r_1)r_2.$

Then the Ore extension R[x;σ,δ] is the noncommutative ring obtained by giving the ring of polynomials R[x] a new multiplication, subject to the identity

$x r=(\sigma r)x + \delta r.$

If δ = 0 (i.e., is the zero map) then the Ore extension is denoted R[x; σ] and is called a skew polynomial ring. If σ = 1 (i.e., the identity map) then the Ore extension is denoted R[x,δ] and is called a differential polynomial ring.

## Examples

The Weyl algebras are Ore extensions, with R any a commutative polynomial ring, σ the identity ring endomorphism, and δ the polynomial derivative.

## Elements

An element f of an Ore ring R is called

• twosided[1] (or invariant[2] ), if R·f = f·R, and
• central, if g·f = f·g for all g ∈ R.