In this paper, we show
how to use the method of Cartan triples (see V. Patrangenaru) in small
dimensions.
We classify the 3-dimensional simply connected homogeneous Riemannian spaces,
and the 4-dimensional simply connected homogeneous Riemannian spaces with
5-dimensional total isometry group. We show that the smallest dimension where
locally homogeneous Riemannian manifolds that are not locally isometric to
homogeneous Riemannian spaces exist, is 5.
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