A novel derivation of the boundary term for the action in Lanczos–Lovelock gravity

We present a novel derivation of the boundary term for the action in Lanczos–Lovelock gravity, starting from the boundary contribution in the variation of the Lanczos–Lovelock action. The derivation presented here is straightforward, i.e., one starts from the Lanczos–Lovelock action principle and the action itself dictates the boundary structure and hence the boundary term one needs to add to the action to make it well-posed. It also gives the full structure of the contribution at the boundary of the complete action, enabling us to read off the degrees of freedom to be fixed at the boundary, their corresponding conjugate momenta and the total derivative contribution on the boundary. We also provide a separate derivation of the Gauss–Bonnet case.