On PP-rings
A ring \(R\) is called a left PP-ring if every principal left ideal is projective. The author gives a new characterization of left PP-rings, uses that to give an elementary proof of a result of the reviewer [Kobe J. Math. 7, No. 1, 77-80 (1990; Zbl 0726.16003)] characterizing triangular PP-rings, and then determines when the ring \(T_ n(R)\) of upper triangular matrices over \(R\) is a left PP-ring.
scientific article; zbMATH DE number 558688