On p.p.-rings which are reduced.
A unital ring \(R\) is (left) right p.p. if every (left) right principal ideal of \(R\) is projective. \(R\) is called p.p. if it is left and right p.p. The main theorem of the paper states that a p.p.-ring contains no nonzero nilpotent elements (i.e., it is reduced) if and only if it has no subrings isomorphic to the \(2\times 2\) upper triangular matrices over the ring of integers, \(\mathbb{Z}\), or to the \(2\times 2\) upper triangular matrices over the fields of residues modulo primes, \(\mathbb{Z}_p\).
scientific article; zbMATH DE number 5163991
scientific article