Comorphic rings

Abstract

A ring [Formula: see text] is called left comorphic if, for each [Formula: see text] there exists [Formula: see text] such that [Formula: see text] and [Formula: see text] Examples include (von Neumann) regular rings, and [Formula: see text] for a prime [Formula: see text] and [Formula: see text] One motivation for studying these rings is that the comorphic rings (left and right) are just the quasi-morphic rings, where [Formula: see text] is left quasi-morphic if, for each [Formula: see text] there exist [Formula: see text] and [Formula: see text] in [Formula: see text] such that [Formula: see text] and [Formula: see text] If [Formula: see text] here the ring is called left morphic. It is shown that [Formula: see text] is left comorphic if and only if, for any finitely generated left ideal [Formula: see text] there exists [Formula: see text] such that [Formula: see text] and [Formula: see text] Using this, we characterize when a left comorphic ring has various properties, and show that if [Formula: see text] is local with nilpotent radical, then [Formula: see text] is left comorphic if and only if it is right comorphic. We also show that a semiprime left comorphic ring [Formula: see text] is semisimple if either [Formula: see text] is left perfect or [Formula: see text] has the ACC on [Formula: see text] After a preliminary study of left comorphic rings with the ACC on [Formula: see text] we show that a quasi-Frobenius ring is left comorphic if and only if every right ideal is principal; if and only if every left ideal is a left principal annihilator. We characterize these rings as follows: The following are equivalent for a ring [Formula: see text] [Formula: see text] is quasi-Frobenius and left comorphic. [Formula: see text] is left comorphic, left perfect and right Kasch. [Formula: see text] is left comorphic, right Kasch, with the ACC on [Formula: see text] [Formula: see text] is left comorphic, left mininjective, with the ACC on [Formula: see text] Some examples of these rings are given.