A property of orthogonal projectors Original Research Article

It is shown that if P1 and P2 are orthogonal projectors, then a product having P1 and P2 as its factors is equal to another such product if and only if P1 and P2 commute, in which case all products involving P1 and P2 reduce to the orthogonal projector P1P2. This is a generalization of a result by Baksalary and Baksalary [Linear Algebra Appl. 341 (2002) 129], with the proof based on a simple property of powers of Hermitian nonnegative definite matrices.