Some conformal and projective scalar invariants of Riemannian manifolds

It is proved that, on any closed oriented Riemannian $n$-manifold, the vector spaces of conformal Killing, Killing, and closed conformal Killing $r$-forms, where $1\le r\le n-1$, have finite dimensions $t_r$, $k_r$, and $p_r$, respectively. The numbers $t_r$ are conformal scalar invariants of the manifold, and the numbers $k_r$ and $p_r$ are projective scalar invariants; they are dual in the sense that $t_r=t_{n-r}$ and $k_r=p_{n-r}$. Moreover, an explicit expression for a conformal Killing $r$-form on a conformally flat Riemannian $n$-manifold is given.