Inequalities related to isotonicity of projection and antiprojection operators
George Isac, Lars‐Erik Persson · Mathematical Inequalities & Applications · 1998
A sharp inequality named "the property of four elements" has recently been proved and studied in [6] and [12]. One particular reason for this is that the inequality is closely related to the isotonicity of the projection operator onto a closed convex set in an ordered Hilbert space.