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Recall that a (unital) ring $R$ is von Neumann regular (VNR) if, for each $x \in R$, there exists $y \in R$ such that $x = xyx$; and unit-regular if such an element $y$ can be taken to be a unit.

Question. Assume $R$ is a VNR (resp., unit-regular) ring, and let $a, b \in R$ be such that the right annihilator $r(a)$ of $a$ equals to the right annihilator $r(b)$ of $b$. Does it follow that $aR = bR$?

It is relatively simple to see that, if $R$ is a VNR ring and $r(a) = r(b)$ for some $a, b \in R$, then $Ra = Rb$: This is true, more generally, for a right principally injective ring (see, e.g., Lemma 5.1 in Nicholson and Yousif's book on quasi-Frobenius rings); and it is a matter of fact that VNR rings are (left and) right principally injective.

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  • $\begingroup$ Does the right annihilator of a mean ax=0 or xa=0. If it's the former way would aR=BR be related to having the same right annihilator $\endgroup$ May 13, 2021 at 12:46
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    $\begingroup$ This doesn't seem true for nxn matrices over a field $\endgroup$ May 13, 2021 at 12:47
  • $\begingroup$ Yes, I mean $r(a) := \{x \in R \colon ax = 0\}$. $\endgroup$ May 13, 2021 at 13:34

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This seems false if $r(a)=\{x\in R\mid ax=0\}$. The ring $M_n(F)$ of $n\times n$ matrices over a field $F$ is a counterexample for $n\geq 2$. Two matrices have the same right annihilator iff they have the same null space and $aM_n(F)=bM_n(F)$ if and only if they have the same image or column space. It is easy to find matrices with the same null space but different column space like take any non-zero vector $v$ and make all rows $v$ versus making the first row $v$ and all other rows zero. It is well known all semisimple rings are unit regular.

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