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Lorentzian geometry is the geometry of Minkowski spacetime, hence essentially of a Euclidean space, but equipped not with the standard Euclidean Riemannian metric of signature $(+,+,+,…,+)$ (which yields Euclidean geometry) but with the pseudo-Riemannian metric of signature $(−,+,+,…,+).$

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A pseudo-Riemannian version of a theorem by Fubini

Guido Fubini, ``Sugli spazii che ammettono un gruppo continuo di movimenti,'' Annali di Mat., ser. 3, 8 (1903) 54.: Let $M$ be a Riemannian manifold of dimension $d\ge 3$. Its isometry group cannot be …
Thomas Schucker's user avatar
1 vote

A pseudo-Riemannian version of a theorem by Fubini

I just found the answer by G. S. Hall (2003) in Class. Quantum Grav. 20 3745. Theorem 8. Let $M$ be a connected smooth paracompact manifold of dimension $n ≥ 3$ admitting a smooth metric $g$ of signat …
Thomas Schucker's user avatar
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On sub-maximally symmetric Riemannian spaces

Is there a 4-dimensional Riemannian manifold with 8-dimensional isometry group? Context: Guido Fubini (Annali di Mat., ser. 3, 8 (1903) 54) shows that the dimension $n$ of the isometry group of a $d$- …
Thomas Schucker's user avatar