Generalized inverses, ideals, and projectors in rings

Abstract

The theory of generalized inverses of matrices and operators is
closely connected with projections, i.e., idempotent (bounded)
linear transformations. We show that a similar situation occurs in
any associative ring $\mathcal{R}$ with a unit $1 \neq 0$. We prove
that generalized inverses in $\mathcal{R}$ are related to idempotent
group endomorphisms $\rho: \mathcal{R} \rightarrow \mathcal{R}$,
called projectors. We use these relations to give characterizations
and existence conditions for $\{1\}$, $\{2\}$, and $\{1,2\}$
inverses with any given principal/annihilator ideals. As a
consequence, we obtain sufficient conditions for any right/left
ideal of $\mathcal{R}$ to be a principal or an annihilator ideal of
an idempotent element of $\mathcal{R}$. We also study some
particular generalized inverses: Drazin and $(b,c)$ inverses, and in
rings with involution, $(e,f)$ Moore-Penrose, $e$-core, $f$-dual
core, $w$-core, $v$-dual core, right $w$-core, left $v$-dual core,
and $(p,q)$ inverses.