### Symmetry, Integrability and Geometry: Methods and Applications (SIGMA)

SIGMA 14 (2018), 081, 28 pages      arXiv:1705.00023      https://doi.org/10.3842/SIGMA.2018.081

### Local Type I Metrics with Holonomy in ${\rm G}_{2}^*$

Anna Fino a and Ines Kath b
a) Dipartimento di Matematica G. Peano, Università di Torino, Via Carlo Alberto 10, Torino, Italy
b) Institut für Mathematik und Informatik, Universität Greifswald, Walther-Rathenau-Str. 47, D-17487 Greifswald, Germany

Received October 24, 2017, in final form July 29, 2018; Published online August 03, 2018

Abstract
By [arXiv:1604.00528], a list of possible holonomy algebras for pseudo-Riemannian manifolds with an indecomposable torsion free ${\rm G}_{2}^*$-structure is known. Here indecomposability means that the standard representation of the algebra on ${\mathbb R}^{4,3}$ does not leave invariant any proper non-degenerate subspace. The dimension of the socle of this representation is called the type of the Lie algebra. It is equal to one, two or three. In the present paper, we use Cartan's theory of exterior differential systems to show that all Lie algebras of Type I from the list in [arXiv:1604.00528] can indeed be realised as the holonomy of a local metric. All these Lie algebras are contained in the maximal parabolic subalgebra $\mathfrak p_1$ that stabilises one isotropic line of ${\mathbb R}^{4,3}$. In particular, we realise $\mathfrak p_1$ by a local metric.

Key words: holonomy; pseudo-Riemannian manifold; exterior differential system; torsion-free G-structures.

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