New invariants of G₂-structures

الأصل · EN

We define a Z/48-valued homotopy invariant nu of a G₂-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 G₂ obtained by the twisted connected sum construction, the associated torsion-free G₂-structure always has nu = 24. Some holonomy G₂ examples constructed by Joyce by desingularising orbifolds have odd nu. We define a further homotopy invariant xi of G₂-structures such that if M is 2-connected then the pair (nu, xi) determines a G₂-structure up to homotopy and diffeomorphism. The class of a G₂-structure is determined by nu on its own when the greatest divisor of p₁(M) modulo torsion divides 224; this sufficient condition holds for many twisted connected sum G₂-manifolds. We also prove that the parametric h-principle holds for coclosed G₂-structures.