General pseudoadditivity of composable entropy prescribed by existence   of equilibrium
  The concept of composability states that entropy of the total system composed of independent subsystems is a function of entropies of the subsystems. Here, the most general pseudoadditivity rule for composable entropy is derived based only on existence of equilibrium. 
Sumiyoshi Abe (College of Science and Technology, Nihon University,   Funabashi, Chiba, Japan)