Computing $\hat{I}½$-invariants of Joyce's compact $G_2$-manifolds

Abstract

Crowley and Nordström introduced an invariant of $G_2$-structures on the tangent bundle of a closed 7-manifold, taking values in the integers modulo 48. Using the spectral description of this invariant due to Crowley, Goette and Nordström, we compute it for many of the closed torsion-free $G_2$-manifolds defined by Joyce's generalized Kummer construction.

20 pages, comments welcome