The centre of a ring/group $R$ 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.
In the particular case of a unital algebra over a field, the center contains the ground field (see
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Author of the notes: Antonio J. Pan-Collantes
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