Center (ring theory)

In algebra, the centre of a ring R is the subring consisting of the elements x such that xy = yx for all elements y in R. It is a commutative ring and is denoted as Z(R); 'Z' stands for the German word Zentrum, meaning "centre".

If R is a ring, then R is an associative algebra over its centre. Conversely, if R is an associative algebra over a commutative subring S, then S is a subring of the center of R, and if S happens to be the center of R, then the algebra R is called a central algebra.

Examples

• The centre of a commutative ring R is R itself.
• The centre of a skew-field is a field.
• The centre of the (full) matrix ring with entries in a commutative ring R consists of R-scalar multiples of the identity matrix.
• Let F be a field extension of a field k, and R an algebra over k. Then Z(R ⊗_k F) = Z(R) ⊗_k F.
• The centre of the universal enveloping algebra of a Lie algebra plays an important role in the representation theory of Lie algebras. For example, a Casimir element is an element of such a centre that is used to analyze Lie algebra representations. See also: Harish-Chandra isomorphism.
• The centre of a simple algebra is a field.

See also

• Center of a group
• Central simple algebra
• Morita equivalence

References

• Bourbaki, Algebra
• Pierce, Richard S. (1982), Associative algebras, Graduate texts in mathematics, vol. 88, Springer-Verlag, ISBN 978-0-387-90693-5