Closed $\mathrm{G}_2$-eigenforms and exact $\mathrm{G}_2$-structures

TL;DR

This paper investigates the existence and classification of exact G2-structures on certain Lie groups and solvmanifolds, showing non-existence results for closed G2-eigenforms and providing classifications for specific Lie algebra cases.

Contribution

It proves non-existence of invariant closed G2-eigenforms on certain Lie groups and classifies seven-dimensional Lie algebras with specific codimension-one ideals admitting exact G2-structures.

Findings

No invariant closed G2-eigenforms on the studied Lie groups.

Classification of seven-dimensional Lie algebras with complex Heisenberg ideal admitting exact G2-structures.

Identification of nilpotent Lie algebras admitting exact SL(3,C)-structures or half-flat SU(3)-structures.

Abstract

A study is made of left-invariant $\mathrm{G}_2$-structures with an exact 3-form on a Lie group $G$ whose Lie algebra $\mathfrak{g}$ admits a codimension-one nilpotent ideal $\mathfrak{h}$. It is shown that such a Lie group $G$ cannot admit a left-invariant closed $\mathrm{G}_2$-eigenform for the Laplacian and that any compact solvmanifold $\Gamma\backslash G$ arising from $G$ does not admit an (invariant) exact $\mathrm{G}_2$-structure. We also classify the seven-dimensional Lie algebras $\mathfrak{g}$ with codimension-one ideal equal to the complex Heisenberg Lie algebra which admit exact $\mathrm{G}_2$-structures with or without special torsion. To achieve these goals, we first determine the six-dimensional nilpotent Lie algebras $\mathfrak{h}$ admitting an exact $\mathrm{SL}(3,\mathbb{C})$-structure $\rho$ or a half-flat $\mathrm{SU}(3)$-structure $(\omega,\rho)$ with exact $\rho$, respectively.