Extended black hole geometrothermodynamics

Although ordinary laboratory thermodynamic systems are known to be homogeneous systems, black holes are different and cannot be considered within this class. Using the formalism of geometrothermodynamics, we show that black holes should be considered as quasi-homogeneous systems. As a consequence, we argue that coupling constants in generalized gravity theories should be considered as thermodynamic variables, giving raise to extended versions of black hole thermodynamics.