A conjectural formula for $DR_g(a,-a) \lambda_g$

Abstract

We propose a conjectural formula for $DR_g(a,-a) \lambda_g$ and check all its expected properties. Our formula refines the one point case of a similar conjecture made by the first named author in collaboration with Gu\'er\'e and Rossi, and we prove that the two conjectures are in fact equivalent, though in a quite non-trivial way.