13C99 16D80 16U80 2-primal rings Rickart modules \(\delta\)-semiperfect modules \(\delta\)-supplemented modules abelian modules central Armendariz rings central reduced rings central reversible rings central semicommutative rings central symmetric rings principally \(\delta\)-semiperfect modules principally \(\delta\)-supplemented modules principally projective modules reduced modules rigid modules semicommutative modules semiperfect modules supplemented modules symmetric modules symmetric rings