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The subset of a ring, group or semi-group $R$ consisting of elements that commute (are interchangable) with all elements of a certain set $X\subseteq R$; the centralizer of $S$ in $R$ is denoted by $C_R(S)$. The centralizer of an irreducible subring (that is, one not stabilizing proper subgroups) of endomorphisms of an Abelian group in the ring of all endomorphisms of this group is a division ring (Schur's lemma).

====References====
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  N. Jacobson,   "Structure of rings" , Amer. Math. Soc.  (1956)</TD></TR></table>

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