Half-Spin Tautological Relations and Faber's Proportionalities of Kappa Classes

Abstract

We employ the $1/2$-spin tautological relations to provide a particular combinatorial identity. We show that this identity is a statement equivalent to Faber's formula for proportionalities of kappa-classes on ${\mathcal{M}}_g$, $g\ge 2$. We then prove several cases of the combinatorial identity, providing a new proof of Faber's formula for those cases.