文摘
In this work, we consider a Riemannian manifold \(M\) with an almost quaternionic structure \(V\) defined by a three-dimensional subbundle of \((1,1)\) tensors \(F\), \(G\), and \(H\) such that \(\{F,G,H\}\) is chosen to be a local basis for \(V\). For such a manifold there exits a subbundle \(\mathcal{{H}} (M)\) of the bundle of orthonormal frames \(\mathcal{{O}}(M)\). If \(M\) admits a torsion-free connection reducible to a connection in \(\mathcal{{H}}(M)\), then we give a condition such that the torsion tensor of the bundle vanishes. We also prove that if \(M\) admits a torsion-free connection reducible to a connection in \(\mathcal{{H}}(M)\), then the tensors \(\widetilde{F}^2\), \(\widetilde{G}^2\), and \(\widetilde{H}^2\) are torsion-free, that is, they are integrable. Here \(\widetilde{F}\), \(\widetilde{G}\), \(\widetilde{H}\) are the extended tensors of \(F\), \(G\), and \(H\) defined on \(M\). Finally, we show that if the torsions of \(\widetilde{F}^2\), \(\widetilde{G}^2\) and \(\widetilde{H}^2\) vanish, then \(M\) admits a connection with torsion which is reducible to \(\mathcal{{H}}(M)\), and this means that \(\widetilde{F}^2\), \(\widetilde{G}^2\), and \(\widetilde{H}^2\) are integrable. Keywords Subbundle Almost complex structure Almost quaternionic structure Torsion tensor