Xu, W (reprint author), Huazhong Univ Sci & Technol, Sch Phys, Wuhan 430074, Peoples R China.
Based on entropy relations, we derive the thermodynamic bound for entropy and the area of horizons for a Schwarzschild-dS black hole, including the event horizon, Cauchy horizon, and negative horizon (i.e., the horizon with negative value), which are all geometrically bound and comprised by the cosmological radius. We consider the first derivative of the entropy relations to obtain the first law of thermodynamics for all horizons. We also obtain the Smarr relation for the horizons using the scaling discussion. For the thermodynamics of all horizons, the cosmological constant is treated as a thermodynamic variable. In particular, the thermodynamics of the negative horizon are defined well in the r<0 side of space-time. This formula appears to be valid for three-horizon black holes. We also generalize the discussion to thermodynamics for the event horizon and Cauchy horizon of Gauss-Bonnet charged flat black holes because the Gauss-Bonnet coupling constant is also considered to be thermodynamic variable. These results provide further insights into the crucial role played by the entropy relations of multi-horizons in black hole thermodynamics as well as improving our understanding of entropy at the microscopic level. (C) 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license.