Quadruples of orthoprojectors connected by a linear relationship

Abstract

In the explicit form, we deduce formulas for all quadruples of orthoprojectors $P_1, P_2, P_3$, and $P_4$ irreducible to within unitary equivalence and connected by the linear relationship $α_1 P_1 + α_2 P_2 + α_3 P_3 + α_4 P_4 = λ I$, where $(α_1, α_2, α_3, α_4) ∈ ℝ^{+}$.