# Centre of a ring

The centre of a ring $R$ is the collection $Z$ of all elements of the ring $R$ that commute with every element, that is, $$Z=\{z: az = za \textrm{ for all }a \in R\}.$$ The centre of a ring is a subring containing together with every invertible element its inverse. The centre of a ring that is an algebra with a unit element over a field contains the ground field (see Central algebra).