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On the geometry of some solvable extensions of the Heisenberg group. (English) Zbl 06986968
Summary: In this paper we first classify left-invariant generalized Ricci solitons on some solvable extensions of the Heisenberg group in both Riemannian and Lorentzian cases. Then we obtain the exact form of all left-invariant unit time-like vector fields which are spatially harmonic. We also calculate the energy of an arbitrary left-invariant vector field $$X$$ on these spaces and obtain all vector fields which are critical points for the energy functional restricted to vector fields of the same length. Furthermore, we determine all homogeneous Lorentzian structures and their types on these spaces and give a complete and explicit description of all parallel and totally geodesic hypersurfaces of these spaces. The non-existence of harmonic maps in the non-abelian case is proved and it is shown that the existence of Einstein, Einstein-like metrics and some equations in the Riemannian case can not be extended to their Lorentzian analogues.

##### MSC:
 53C30 Differential geometry of homogeneous manifolds 53C50 Global differential geometry of Lorentz manifolds, manifolds with indefinite metrics 53C43 Differential geometric aspects of harmonic maps
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