On the number of terms in the Lovelock products

In this short note we wonder about the explicit expression of the expanding of the p-th Lovelock product. We use the 1990s' works of S. A. Fulling et al. on the symmetries of the Riemann tensor, and we show that the number of independent scalars appearing in this expanding is equal to the number of Young diagrams with all row lengths even in the decomposition of the p-th plethysm of the Young diagram representing the symmetries of the Riemann tensor.