Strongly subadditive functions

Abstract

Let f be a function defined on positive numbers. The subject is the trace inequality $Tr f(A) + Tr f(P_2AP_2) \le Tr f(P_{12}AP_{12}) + \Tr f(P_{23}AP_{23})$, where $A$ is a positive operator, $P_1,P_2,P_3$ are orthogonal projections such that $P_1+P_2+P_3=I$, $P_{12}=P_1+P_2$ and $P_{23}=P_2+P_3$. There are several examples of functions f satisfying the inequality (called (SSA)) and the case of equality is described.