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V. N. Berestovskii

Publications and source records attributed to V. N. Berestovskii.

14 recordsLinked to original sources
arXiv2026

Homogeneous spaces with geodesic orbit Riemannian metrics and with integrable invariant distributions

We consider two types of homogeneous spaces of Lie groups with compact stabilizer subgroups: those with integrable invariant distributions and those for which all invariant Riemannian metrics are geodesic orbit. Recall that a Riemannian metric is called geodesic orbit if each geodesic is an orbit of a one-parameter subgroup of the full isometry group. We exhibit several homogeneous spaces of the first type that do not belong to the second one.

math.DG↗
arXiv2024

The Gödel Universe as a Lie group with left-invariant Lorentz metric and\newline the Iwasawa decomposition

We discuss models of the Gödel Universe as Lie groups with left-invariant Lorentz metric for two simply connected four-dimensional Lie groups, the Iwasawa decomposition for semisimple Lie groups, and left-invariant Lorentz metric on ${\rm SL}(2,\mathbb{R})$, following K.-H.~Neeb. Also we show that the isometry between two non-isomorphic sub-Riemannian Lie group, constructed by A.~Agrachev and D.~Barilari, is induced by some Iwasawa decomposition of ${\rm SL}(2,\mathbb{R})$.

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arXiv2024

The Gödel Universe as the Lie Group with left-invariant Lorentz metric

The author studies the Gödel Universe as the Lie group with left-invariant Lorentz metric. The expressions for timelike and isotropic geodesics in elementary functions are found by methods of geometric theory of optimal control for the search of geodesics on Lie groups with left-invariant (sub-)Lorentz metrics. It is proved that the Gödel Universe has no closed timelike or isotropic geodesics.

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arXiv2023

Perfect and almost perfect homogeneous polytopes

The paper is devoted to perfect and almost perfect homogeneous polytopes in Euclidean spaces. We classified perfect and almost perfect polytopes among all regular polytopes and all semiregular polytopes excepting Archimedean solids and two four-dimensional Gosset polytopes. Also we construct some non-regular homogeneous polytopes that are (or are not) perfect and posed some unsolved questions.

math.MG↗
arXiv2018

On homogeneous geodesics and weakly symmetric spaces

In this paper, we establish a sufficient condition for a geodesic in a Riemannian manifold to be homogeneous, i.e. an orbit of an $1$-parameter isometry group. As an application of this result, we provide a new proof of the fact that every weakly symmetric space is geodesic orbit manifold, i.e. all its geodesics are homogeneous. We also study general properties of homogeneous geodesics, in particular, the structure of the closure of a given homogeneous geodesic. We present several examples where this closure is a torus of dimension $\geq 2$ which is (respectively, is not) totally geodesic in the ambient manifold. Finally, we discuss homogeneous geodesics in Lie groups supplied with left-invariant Riemannian metrics.

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arXiv2014

Locally compact homogeneous spaces with inner metric

The author reviews his results on locally compact homogeneous spaces with inner metric, in particular, homogeneous manifolds with inner metric. The latter are isometric to homogeneous (sub-)Finslerian manifolds; under some additional conditions they are isometric to homogeneous (sub)-Riemannian manifolds. The class $Ω$ of all locally compact homogeneous spaces with inner metric is supplied with some metric $d_{BGH}$ such that 1) $(Ω,d_{BGH})$ is a complete metric space; 2) a sequences in $(Ω,d_{BGH})$ is converging if and only if it is converging in Gromov-Hausdorff sense; 3) the subclasses $\mathfrak{M}$ of homogeneous manifolds with inner metric and $\mathfrak{LG}$ of connected Lie groups with left-invariant Finslerian metric are everywhere dense in $(Ω,d_{BGH}).$ It is given a metric characterization of Carnot groups with left-invariant sub-Finslerian metric. At the end are described homogeneous manifolds such that any invariant inner metric on any of them is Finslerian.

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arXiv2012

Generalized normal homogeneous Riemannian metrics on spheres and projective spaces

In this paper we develop new methods of study of generalized normal homogeneous Riemannian manifolds. In particular, we obtain a complete classification of generalized normal homogeneous Riemannian metrics on spheres. We prove that for any connected (almost effective) transitive on $S^n$ compact Lie group $G$, the family of $G$-invariant Riemannian metrics on $S^n$ contains generalized normal homogeneous but not normal homogeneous metrics if and only if this family depends on more than one parameters. Any such family (that exists only for $n=2k+1$) contains a metric $g_{\can}$ of constant sectional curvature 1 on $S^n$. We also prove that $(S^{2k+1}, g_{\can})$ is Clifford-Wolf homogeneous, and therefore generalized normal homogeneous, with respect to $G$ (excepting the groups $G=SU(k+1)$ with odd $k+1$). The space of unit Killing vector fields on $(S^{2k+1}, g_{\can})$ from Lie algebra $\mathfrak{g}$ of Lie group $G$ is described as some symmetric space (excepting the case $G=U(k+1)$ when one obtains the union of all complex Grassmannians in $\mathbb{C}^{k+1}$).

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arXiv2009

Covering R-trees, R-free groups, and dendrites

We prove that every length space X is the orbit space (with the quotient metric) of an R-tree T via a free action of a locally free subgroup G(X) of isometries of X. The mapping f:T->X is a kind of generalized covering map called a URL-map and is universal among URL-maps onto X. T is the unique R-tree admitting a URL-map onto X. When X is a complete Riemannian manifold M of dimension n>1, the Menger sponge, the Sierpin'ski carpet or gasket, T is isometric to the so-called "universal" R-tree A_{c}, which has valency equal to the cardinality of the continuum at each point. In these cases, and when X is the Hawaiian earring H, the action of G(X) on T gives examples in addition to those of Dunwoody and Zastrow that negatively answer a question of J. W. Morgan about group actions on R-trees. Indeed, for one length metric on H, we obtain precisely Zastrow's example.

math.MG↗
arXiv2009

The classification of $δ$-homogeneous Riemannian manifolds with positive Euler characteristic

The authors give a short survey of previous results on $δ$-homogeneous Riemannian manifolds, forming a new proper subclass of geodesic orbit spaces with non-negative sectional curvature, which properly includes the class of all normal homogeneous Riemannian manifolds. As a continuation and an application of these results, they prove that the family of all compact simply connected indecomposable $δ$-homogeneous Riemannian manifolds with positive Euler characteristic, which are not normal homogeneous, consists exactly of all generalized flag manifolds $Sp(l)/U(1)\cdot Sp(l-1)=\mathbb{C}P^{2l-1}$, $l\geq 2$, supplied with invariant Riemannian metrics of positive sectional curvature with the pinching constants (the ratio of the minimal sectional curvature to the maximal one) in the open interval $(1/16, 1/4)$. This implies very unusual geometric properties of the adjoint representation of $Sp(l)$, $l\geq 2$. Some unsolved questions are suggested.

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arXiv2008

Covering R-trees

We show that every inner metric space X is the metric quotient of a complete R-tree via a free isometric action, which we call the covering R-tree of X. The quotient mapping is a weak submetry (hence, open) and light. In the case of compact 1-dimensional geodesic space X, the free isometric action is via a subgroup of the fundamental group of X. In particular, the Sierpin'ski gasket and carpet, and the Menger sponge all have the same covering R-tree, which is complete and has at each point valency equal to the continuum. This latter R-tree is of particular interest because it is "universal" in at least two senses: First, every R-tree of valency at most the continuum can be isometrically embedded in it. Second, every Peano continuum is the image of it via an open light mapping. We provide a sketch of our previous construction of the uniform universal cover in the special case of inner metric spaces, the properties of which are used in the proof.

math.MG↗
arXiv2008

Clifford-Wolf homogeneous Riemannian manifolds

In this paper, using connections between Clifford-Wolf isometries and Killing vector fields of constant length on a given Riemannian manifold, we classify simply connected Clifford-Wolf homogeneous Riemannian manifolds. We also get the classification of complete simply connected Riemannian manifolds with the Killing property defined and studied previously by J.E. D'Atri and H.K. Nickerson. In the last part of the paper we study properties of Clifford-Killing spaces, that is, real vector spaces of Killing vector fields of constant length, on odd-dimensional round spheres, and discuss numerous connections between these spaces and various classical objects.

math.DG↗
arXiv2007

On $δ$-homogeneous Riemannian manifolds

We study in this paper previously defined by V.N. Berestovskii and C.P. Plaut $δ$-homogeneous spaces in the case of Riemannian manifolds. Every such manifold has non-negative sectional curvature. The universal covering of any $δ$-homogeneous Riemannian manifolds is itself $δ$-homogeneous. In turn, every simply connected Riemannian $δ$-homogeneous manifold is a direct metric product of an Euclidean space and compact simply connected indecomposable homogeneous manifolds; all factors in this product are itself $δ$-homogeneous. We find different characterizations of $δ$-homogeneous Riemannian spaces, which imply that any such space is geodesic orbit (g.o.) and every normal homogeneous Riemannian manifold is $δ$-homogeneous. The g.o. property and the $δ$-homogeneity property are inherited by closed totally geodesic submanifolds. Then we find all possible candidates for compact simply connected indecomposable Riemannian $δ$-homogeneous non-normal manifolds of positive Euler characteristic and a priori inequalities for parameters of the corresponding family of Riemannian $δ$-homogeneous metrics on them (necessarily two-parametric). We prove that there are only two families of possible candidates: non-normal (generalized) flag manifolds $SO(2l+1)/U(l)$ and $Sp(l)/U(1)\cdot Sp(l-1)$, $l\geq 2$, investigated earlier by W. Ziller, H. Tamaru, D.V. Alekseevsky and A. Arvanitoyeorgos. At the end we prove that the corresponding two-parametric family of Riemannian metrics on $SO(5)/U(2)=Sp(2)/U(1)\cdot Sp(1)$ satisfying the above mentioned (strict!) inequalities, really generates $δ$-homogeneous spaces, which are not normal and are not naturally reductive with respect to any isometry group.

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arXiv2006

Killing vector fields of constant length on Riemannian manifolds

In this paper nontrivial Killing vector fields of constant length and corresponding flows on smooth complete Riemannian manifolds are investigated. It is proved that such a flow on symmetric space is free or induced by a free isometric action of the circle $S^1$. The properties of the set of all points with finite (infinite) period for general isometric flow on Riemannian manifolds are described. It is shown that this flow is generated by an effective almost free isometric action of the group $S^1$ if there are no points of infinite or zero period. In the last case the set of periods is at most countable and naturally generates an invariant stratification with closed totally geodesic strata; the union of all regular orbits is open connected everywhere dense subset of complete measure. Examples of unit Killing vector fields generated by almost free but not free actions of $S^1$ on Riemannian manifolds close in some sense to symmetric spaces are constructed; among them are "almost round" odd-dimensional spheres, homogeneous (non simply connected) Riemannian manifolds of constant positive sectional curvature, locally Euclidean spaces, and unit vector bundles over Riemannian manifolds. Some curvature restrictions on Riemannian manifolds admitting nontrivial Killing vector fields of constant length are obtained. Some unsolved questions are formulated.

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