G2-structure
{{DISPLAYTITLE:G2-structure}} In differential geometry, a -structure is an important type of G-structure that can be defined on a smooth manifold. If M is a smooth manifold of dimension seven, then a G2-structure is a reduction of structure group of the frame bundle of M to the compact, exceptional Lie group G2. Equivalent conditions The condition of M admitting a structure is equivalent to any of the following conditions: The first and second Stiefel–Whitney classes of M vanish. M is orientable and admits a spin structure. The last condition above correctly suggests that many manifolds admit -structures. History A manifold with holonomy was first introduced by Edmond Bonan in 1966, who constructed the parallel 3-form, the parallel 4-form and showed that this manifold was Ricci-flat. The first complete, but noncompact 7-manifolds with holonomy were constructed by Robert Bryant and Salamon in 1989. The first compact 7-manifolds with holonomy were constructed by Dominic Joyce in 1994, and compact manifolds are sometimes known as "Joyce manifolds", especially in the physics literature. In 2013, it was shown by M. Firat Arikan, Hyunjoo Cho, and Sema Salur that any manifold with a spin structure, and, hence, a -structure, admits a compatible almost contact metric structure,