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Yoshinobu Kamishima

Publications and source records attributed to Yoshinobu Kamishima.

15 recordsLinked to original sources
arXiv2023

On the automorphism group of parabolic structures and closed aspherical manifolds

In this expository paper we discuss several properties on closed aspherical parabolic ${\sfG}$-manifolds $X/Γ$. These are manifolds $X/Γ$, where $X$ is a smooth contractible manifold with a parabolic ${\sfG}$-structure for which $Γ\leq \Aut_{\sfG}(X)$ is a discrete subgroup acting properly discontinuously on $X$ with compact quotient. By a parabolic $\sfG$-structure on $X$ we have in mind a Cartan structure which is modeled on one of the classical parabolic geometries arising from simple Lie groups $\sfG$ of rank one. Our results concern in particular the properties of the automorphism groups $\Aut_{\sfG}(X/Γ)$. Our main results show that the existence of certain parabolic ${\sfG}$-structures can pose strong restrictions on the topology of compact aspherical manifolds $X/Γ$ and their parabolic automorphism groups. In this realm we prove that any compact aspherical standard $CR$-manifold with virtually solvable fundamental group is diffeomorphic to a quotient of a Heisenberg manifold of complex type with its standard $CR$-structure. Furthermore we discuss the analogue properties of standard quaternionic contact manifolds in relation to the quaternionic Heisenberg group.

math.DG↗
arXiv2022

Quaternionic contact structure with integrable complementary distribution

We study positive definite quaternionic contact $(4n+3)$-manifolds ($qc$-manifold for short). Just like the $CR$-structure contains the class of Sasaki manifolds, the $qc$-structure admits a class of $3$-Sasaki manifolds with integrable distribution isomorphic to $\mathfrak{su}(2)$. A big difference concerning the integrable complementary $qc$-distribution $V$ of the $qc$-structure from $3$-Sasaki structure is the existence of Lie algebra not isomorphic to $\mathfrak{su}(2)$. We take up non-compact $qc$-manifolds to find out a salient feature of topology and geometry in case $V$ generates the $qc$-transformations $R^3$.

math.GT↗
arXiv2021

A note on vanishing of equivariant differentiable cohomology of proper actions and application to CR-automorphism and conformal groups

We establish that for any proper action of a Lie group on a manifold the associated equivariant differentiable cohomology groups with coefficients in modules of $\mathcal{C}^\infty$-functions vanish in all degrees except than zero. Furthermore let $G$ be a Lie group of $CR$-automorphisms of a strictly pseudo-convex $CR$-manifold $M$. We associate to $G$ a canonical class in the first differential cohomology of $G$ with coefficients in the $\mathcal{C}^\infty$-functions on $M$. This class is non-zero if and only if $G$ is essential in the sense that there does not exist a $CR$-compatible strictly pseudo-convex pseudo-Hermitian structure on $M$ which is preserved by $G$. We prove that a closed Lie subgroup $G$ of $CR$-automorphisms acts properly on $M$ if and only if its canonical class vanishes. As a consequence of Schoen's theorem, it follows that for any strictly pseudo-convex $CR$-manifold $M$, there exists a compatible strictly pseudo-convex pseudo-Hermitian structure such that the CR-automorphism group for $M$ and the group of pseudo-Hermitian transformations coincide, except for two kinds of spherical $CR$-manifolds. Similar results hold for conformal Riemannian and Kähler manifolds.

math.DG↗
arXiv2020

Homogeneous Sasaki and Vaisman manifolds of unimodular Lie groups

A Vaisman manifold is a special kind of locally conformally Kaehler manifold, which is closely related to a Sasaki manifold. In this paper we show a basic structure theorem of simply connected homogeneous Sasaki and Vaisman manifods of unimodular Lie groups, up to holomorphic isometry. For the case of unimodular Lie groups, we obtain a complete classification of simply connected Sasaki and Vaisman unimodular Lie groups, up to modification.

math.DG↗
arXiv2019

Locally Homogeneous Aspherical Sasaki Manifolds

Let $G/H$ be a contractible homogeneous Sasaki manifold. A compact locally homogeneous aspherical Sasaki manifold $Γ\big\backslash G/H$ is by definition a quotient of $G/H$ by a discrete uniform subgroup $Γ\leq G$. We show that a compact locally homogeneous aspherical Sasaki manifold is always quasi-regular, that is, $Γ\big\backslash G/H$ is an $S^{1}$-Seifert bundle over a locally homogeneous aspherical Kähler orbifold. We discuss the structure of the isometry group $\mathrm{Isom}(G/H)$ for a Sasaki metric of $G/H$ in relation with the pseudo-Hermitian group $\mathrm{Psh} (G/H)$ for the Sasaki structure of $G/H$. We show that a Sasaki Lie group $G$, when $Γ\big\backslash G$ is a compact locally homogeneous aspherical Sasaki manifold, is either the universal covering group of $SL(2,R)$ or a modification of a Heisenberg nilpotent Lie group with its natural Sasaki structure. In addition, we classify all aspherical Sasaki homogeneous spaces for semisimple Lie groups.

math.DG↗
arXiv2018

Isometry groups with radical, and aspherical Riemannian manifolds with large symmetry I

Every compact aspherical Riemannian manifold admits a canonical series of orbibundle structures with infrasolv fibers which is called its infrasolv tower. The tower arises from the solvable radicals of isometry group actions on the universal covers. Its length and the geometry of its base measure the degree of continuous symmetry of an aspherical Riemannian manifold. We say that the manifold has large symmetry if it admits an infrasolv tower whose base is a locally homogeneous space. We construct examples of aspherical manifolds with large symmetry, which do not support any locally homogeneous Riemannian metrics.

math.DG↗
arXiv2015

Homogeneous locally conformally Kaehler and Sasaki manifolds

We prove various classification results for homogeneous locally conformally symplectic manifolds. In particular, we show that a homogeneous locally conformally Kaehler manifold of a reductive group is of Vaisman type, if the normalizer of the isotropy group is compact. We also show that such a result does not hold in the case of noncompact normalizer and determine all left-invariant locally conformally Kaehler structures on reductive Lie groups.

math.DG↗
arXiv2014

Locally conformally Kaehler structures on homogeneous spaces

We will discuss in this paper homogeneous locally conformally Keahler (or shortly homogeneous l.c.K.) manifolds and locally homogeneous l.c.K. manifolds from various aspects of study in the field of l.c.K. geometry. We will provide a survey of known results along with some new results and observations; in particular we make a complete classification of 4-dimensional homogeneous and locally homogeneous l.c.K. manifolds in terms of Lie algebras.

math.DG↗
arXiv2013

Compact Homogeneous Locally Conformally Kaehler Manifolds

In this paper we show as main results two structure theorems of a compact homogeneous locally conformally Kaehler (or shortly l.c.K.) manifold, a holomorphic structure theorem asserting that it has a structure of holomorphic principal fiber bundle over a flag manifold with fiber a 1-dimensional complex torus, and a metric structure theorem asserting that it is necessarily of Vaisman type. We also discuss and determine l.c.K. reductive Lie groups and compact locally homogeneous l.c.K. manifolds of reductive Lie groups.

math.CV↗
arXiv2011

Lorentzian similarity manifold

If an $m+2$-manifold $M$ is locally modeled on $\RR^{m+2}$ with coordinate changes lying in the subgroup $G=\RR^{m+2}\rtimes ({\rO}(m+1,1)\times \RR^+)$ of the affine group ${\rA}(m+2)$, then $M$ is said to be a \emph{Lorentzian similarity manifold}. A Lorentzian similarity manifold is also a conformally flat Lorentzian manifold because $G$ is isomorphic to the stabilizer of the Lorentz group ${\rPO}(m+2,2)$ which is the full Lorentzian group of the Lorentz model $S^{2n+1,1}$. It contains a class of Lorentzian flat space forms. We shall discuss the properties of compact Lorentzian similarity manifolds using developing maps and holonomy representations.

math.GT↗
arXiv2010

Conformally flat Lorentz manifolds and Fefferman Lorentz metrics

We study conformal Fefferman-Lorentz manifolds introduced by Fefferman. To do so, we introduce Fefferman-Lorentz structure on (2n+2)-dimensional manifolds. By using causal conformal vector fields preserving that structure, we shall establish two theorems on compact Fefferman-Lorentz manifolds: One is the coincidence of vanishing curvature between Weyl conformal curvature tensor of Fefferman metrics on a Lorentz manifold $S^1\times N$ and Chern-Moser curvature tensor on a strictly pseudoconvex CR-manifold $N$. Another is the analogue of the conformal rigidity theorem of Obata and Lelong to the compact Fefferman-Lorentz manifolds admitting noncompact closed causal conformal Fefferman-Lorentz transformations.

math.DG↗
arXiv2008

Cohomological rigidity of real Bott manifolds

A real Bott manifold is the total space of iterated RP^1 bundles starting with a point, where each RP^1 bundle is projectivization of a Whitney sum of two real line bundles. We prove that two real Bott manifolds are diffeomorphic if their cohomology rings with Z/2 coefficients are isomorphic. A real Bott manifold is a real toric manifold and admits a flat riemannian metric invariant under the natural action of an elementary abelian 2-group. We also prove that the converse is true, namely a real toric manifold which admits a flat riemannian metric invariant under the action of an elementary abelian 2-group is a real Bott manifold.

math.AT↗
arXiv2006

Nonexistence of cusp cross-section of one-cusped complete complex hyperbolic manifolds II

D. D. Long and A. W. Reid have shown that some compact flat 3-manifold cannot be diffeomorphic to a cusp cross-section of any complete finite volume 1-cusped real hyperbolic 4-manifold. This note concerns the complex hyperbolic case. We give a negative answer that there exists a 3-dimensional closed Heisenberg infranilmanifold which cannot be diffeomorphic to a cusp cross-section of any complete finite volume 1-cusped complex hyperbolic 2-manifold.

math.AT↗
arXiv2000

Transformations of compact locally conformally Kähler manifolds

We characterize compact locally conformally Kähler (l.c.K.) manifolds under the assumption of a purely conformal, holomorphic circle action. As an application, we determine the structure of the compact l.c.K. manifolds with parallel Lee form. We introduce the Lee-Cauchy-Riemann (LCR) transformations as a class of diffeomorphisms preserving the specific $G$-structure of l.c.K. manifolds. Then we characterize the Hopf manifolds, up to holomorphic isometry, as compact l.c.K. manifolds admitting a certain closed LCR action of $\mathbb{C}^*$.

math.DG↗