The only possible second-order Euler–Lagrange expression obtainable in a four-dimensional space from a scalar density of the form L = L ( g μ μ ν ν ) is E μ μ ν ν = α α − − g [ R μ μ ν ν − − 1 2 g μ μ ν ν R ] + λ λ − − g g μ μ ν ν