The difference and sum of projectors Original Research Article

The aim of this paper is to present new results on the invertibility of the sum of projectors, and new relations between the nonsingularity of the difference and the sum of projectors. We also give simple proofs, without referring to rank theory, of some results on the nonsingularity of the difference of projectors obtained by Groß and Trenkler [SIAM J. Matrix Anal. Appl. 21 (1999) 390].