Nonadditive thermostatistics and thermodynamics
  Nonadditive composition rules for several physical quantities are treated in thermodynamics. It is argued that the zeroth law defines the existence of their additive forms, the formal logarithms. A further principle, the universal thermostat independence leads to a particular formal logarithm, equivalent to Tsallis entropy S_q. We connect q with generalized susceptibilities of the thermostat. 
P. V\'an, G. G. Barnaf\"oldi, T. S. Bir\'o and K \"Urm\"ossy
cond-mat.stat-mech nucl-th