Groß and Trenkler [SIAM J. Matrix Anal. Appl. 21 (1999) 390] pointed out that if a difference of idempotent matrices P1 and P2 is nonsingular, then so is their sum, and Koliha
et al. [Linear Algebra Appl., in press] expressed explicitly a condition, which combined with
the nonsingularity of P1 + P2 ensures the nonsingularity of P1 − P2. In the present note, these
results are strengthened by showing that the nonsingularity of P1 + P2 is in fact equivalent
to the nonsingularity of any linear combination c1P1 + c2P2, wherein c1 + c2 =/ 0. Some
other nonsingularity-type relationships referring to linear combinations of P1 and P2 are also