
This paper was inspired by the definition and study of injectivity domains (as in \textit{A.~N. Alahmadi, M. Alkan} and \textit{S. López-Permouth}, [Glasg. Math. J. 52A, 7-17 (2010; Zbl 1228.16004)] and \textit{N. Er, S. López-Permouth} and \textit{N. Sökmez} [J. Algebra 330, No. 1, 404-417 (2011; Zbl 1227.16004)]). The dual concepts are defined here: For any (unital right) \(R\)-module \(M\), the projectivity domain of \(M\) is defined as \(\{N\in\text{Mod-}R:M\) is \(N\)-projective\}. A module \(M\) is said to be projectively poor (p-poor) if its projectivity domain consists of precisely the semisimple \(R\)-modules. If a ring \(R\) is such that every \(R\)-module is either projective or projectively poor, then \(R\) is said to have no (right) p-middle class. Properties of p-poor modules are investigated first. For example, it is shown that if \(M_R\) is a p-poor module, then so is \(M\oplus N\) for all \(N\in\text{Mod-}R\). Restrictions are then placed on the ring. Semisimple Artinian rings are characterized in terms of p-poor \(R\)-modules. It is proved that every ring has a semisimple p-poor module. If \(R\) is semilocal, then \((R/J(R))_R\) is shown to be p-poor and if \(R\) is a right PCI-domain, then \(E(R)\) (and consequently every nonzero injective module) is p-poor. Rings with no (right) p-middle class are studied next. The following results are found for a ring \(R\) with no right p-middle class: (1) For any two-sided ideal \(I\), \(R/I\) is semisimple Artinian or \(I\) is a direct summand of \(R\). (2) If \(J(R)\neq 0\), then \(R\) is semilocal (3) \(\text{Soc}(R_R)\) is a direct summand of \(R\) or \(R\) is semiartinian with Loewy length \(\leq 2\). (4) If \(J(R)\neq J(R)^2\), then \(J(R)=0\). (5) Factor rings of \(R\) have no right p-middle class. (6) \(R\) is either semiprimary with \(J(R)^2=0\) or semiprime. (7) \(R\) does not contain an infinite independent family of nonzero two-sided ideals. (8) \(R\) does not contain an infinite set of central orthogonal idempotents. (9) If \(R\) is also indecomposable, then \(\text{Soc}(R_R)\leq_eR\) with \(J(R)^2=0\) or \(\text{Soc}(R_R)=0\). (10) If \(R\) is indecomposable semiprime, and \(R\) is not a prime ring, then \(R\) is semisimple Artinian. The main theorem states that every ring with no right p-middle class is the ring direct sum of a semisimple Artinian ring \(S\) and a ring \(K\) which is either zero or an indecomposable ring with exactly one of the following properties: (1) \(K\) is a semiprimary right SI-ring with \(J(K)\neq 0\), or (2) \(K\) is a semiprimary ring with \(\text{Soc}(K_K)=Z_r(K)=J(K)\neq 0\), or (3) \(K\) is a prime ring with \(\text{Soc}(K_K)=0\) and either \(J(K)=0\) or \(_KJ(K)\) and \(J(K)_K\) is infinitely generated, or (4) \(K\) is a prime right SI-ring with infinitely generated right socle. Some examples are found of rings that have no right p-middle class, including QF-rings with unique simple module (up to isomorphism) and \(J(R)^2=0\).
Algebra and Number Theory, projective modules, p-middle classes, Free, projective, and flat modules and ideals in associative algebras, Structure and classification for modules, bimodules and ideals (except as in 16Gxx), direct sum decomposition and cancellation in associative algebras), Simple and semisimple modules, primitive rings and ideals in associative algebras, projectively poor modules, semisimple modules, projectivity domains
Algebra and Number Theory, projective modules, p-middle classes, Free, projective, and flat modules and ideals in associative algebras, Structure and classification for modules, bimodules and ideals (except as in 16Gxx), direct sum decomposition and cancellation in associative algebras), Simple and semisimple modules, primitive rings and ideals in associative algebras, projectively poor modules, semisimple modules, projectivity domains
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