Rotation index, Milnor-Munkres-Novikov pairing, and group actions on manifolds
with M. Bustamante
Submitted. arXiv:2408.09346
We introduce an invariant of a pair of commuting invertible matrices that we call the rotation index. We apply this invariant, together with the Milnor-Munkres-Novikov pairing, to the study of some questions about group actions of Z^2, specifically the Nielsen realization problem, higher-rank Anosov actions, and extending actions from spheres to disks.