The First Law of Thermodynamics of the (2+1)-Dimensional Banados--Teitelboim--Zanelli Black Holes and Kerr--de Sitter Spacetimes

We investigate the first law of thermodynamics in the case of the (2+1)-dimensional Banados--Teitelboim--Zanelli black holes and Kerr--de Sitter spacetimes. In particular, we focus on the integral mass formulas. It is found that by assuming the cosmological constant as a variable state parameter, both the differential and integral mass formulas of the first law of black hole thermodynamics in the asymptotic flat spacetimes can be directly extended to those of rotating black holes in anti-de Sitter and de Sitter backgrounds. It should be pointed that these formulae come into existence in any dimensions.