New invariants of G_2-structures

TL;DR

This paper introduces new homotopy invariants for G_2-structures on 7-manifolds, providing tools to classify and distinguish these structures, especially in the context of twisted connected sums and Joyce's orbifold desingularizations.

Contribution

It defines the invariants nu and xi for G_2-structures, linking them to topological properties and establishing conditions for classification up to homotopy and diffeomorphism.

Findings

nu invariant takes value 24 for twisted connected sum G_2-manifolds

Some Joyce orbifold examples have odd nu values

Parametric h-principle proven for coclosed G_2-structures

Abstract

We define a Z/48-valued homotopy invariant nu of a G_2-structure on the tangent bundle of a closed 7-manifold in terms of the signature and Euler characteristic of a coboundary with a Spin(7)-structure. For manifolds of holonomy G_2 obtained by the twisted connected sum construction, the associated torsion-free G_2-structure always has nu = 24. Some holonomy G_2 examples constructed by Joyce by desingularising orbifolds have odd nu. We define a further homotopy invariant xi of G_2-structures such that if M is 2-connected then the pair (nu, xi) determines a G_2-structure up to homotopy and diffeomorphism. The class of a G_2-structure is determined by nu on its own when the greatest divisor of p_1(M) modulo torsion divides 224; this sufficient condition holds for many twisted connected sum G_2-manifolds. We also prove that the parametric h-principle holds for coclosed G_2-structures.