Nearly Parallel \texorpdfstring{$\G_{2}$}{G2}-Structures with Torus Symmetry

Abstract: We study nearly parallel $\G_{2}$-structures with a three-torus symmetry via multi-moment map techniques. An effective three-torus action on a nearly parallel $\G_{2}$-manifold yields a multi-moment map. The torus acts freely on its regular level sets, so they are torus bundles over smooth three-dimensional manifolds. We show that the geometry of the base spaces is specified by two triples of closed two-forms related by a Riemannian metric. We then describe an inverse construction producing invariant nearly parallel $\G_{2}$-structures from three-dimensional data. We observe that locally this may produce examples with four-torus symmetry.