ELA, Volume 15, pp. 154-158, May 2006, abstract.
On Polynomials in Two Projections
Ilya M. Spitkovsky
Conditions are established on a polynomial f in two
variables under which the equality f(P_1,P_2)=0 for
two orthogonal projections P_1, P_2 is possible only
if P_1 and P_2 commute.