Gauss–Bonnet gravity
In general relativity, Gauss–Bonnet gravity, also referred to as Einstein–Gauss–Bonnet gravity, is a modification of the Einstein–Hilbert action to include the Gauss–Bonnet term (named after Carl Friedrich Gauss and Pierre Ossian Bonnet) ∫ d D x − g G {\displaystyle \int d^{D}x{\sqrt {-g}}\,G} , where G = R 2 − 4 R μ ν R μ ν + R μ ν ρ σ R μ ν ρ σ {\displaystyle G=R^{2}-4R^{\mu \nu }R_{\mu \nu }+R^{\mu \nu \rho \sigma }R_{\mu \nu \rho \sigma }} . This term is only nontrivial in 4+1D or greater, and as such, only applies to extra dimensional models.