Let f: ℝ+ → ℝ. The subject is the trace inequality Tr f(A) + Tr f(P2AP2) ≦ Tr f(P12AP12) + Tr f(P23AP23), where A is a positive operator, P1; P2; P3 are orthogonal projections such that P1 + P2 + P3 = I, P12 = P1 + P2 and P23 = P2 + P3. There are several examples of functions f satisfying the inequality (called (SSA)) and the case of equality is described.