Geometric structures of vectorial type

Abstract

We study geometric structures of $\mathcal{W}_4$-type in the sense of A. Gray on a Riemannian manifold. If the structure group \mathrm{G} \subset \SO(n) preserves a spinor or a non-degenerate differential form, its intrinsic torsion $\Gamma$ is a closed 1-form (Proposition \ref{dGamma} and Theorem \ref{Fixspinor}). Using a $\mathrm{G}$-invariant spinor we prove a splitting theorem (Proposition \ref{splitting}). The latter result generalizes and unifies a recent result obtained in \cite{Ivanov&Co}, where this splitting has been proved in dimensions $n=7,8$ only. Finally we investigate geometric structures of vectorial type and admitting a characteristic connection $\nabla^{\mathrm{c}}$. An interesting class of geometric structures generalizing Hopf structures are those with a $\nabla^{\mathrm{c}}$-parallel intrinsic torsion $\Gamma$. In this case, $\Gamma$ induces a Killing vector field (Proposition \ref{Killing}) and for some special structure groups it is even parallel.Comment: 11 pages, Latex2