Semi-Riemannian submersions with totally umbilic fibres

Semi-invariant submersions from almost Hermitian manifolds

We introduce semi-invariant Riemannian submersions from almost Hermitian manifolds onto Riemannian manifolds. We give examples, investigate the geometry of foliations which are arisen from the definition of a Riemannian submersion and find necessary-sufficient conditions for total manifold to be locally product Riemannian manifold. We also find necessary and sufficient conditions for a semi-invariant submersion to be totally geodesic. Moreover, we obtain a classification for semi-invariant submersions with totally umbilical fibers and show that such submersions put some restrictions on total m…

On semi-slant $ξ^\perp-$Riemannian submersions

The aim of the present paper to define and study semi-slant $ξ^\perp-$Riemannian submersions from Sasakian manifolds onto Riemannian manifolds as a generalization of anti-invariant $ξ^\perp-$Riemannian submersions, semi-invariant $ξ^\perp-$Riemannian submersions and slant Riemannian submersions. We obtain characterizations, investigate the geometry of foliations which arise from the definition of this new submersion. After we investigate the geometry of foliations, we obtain necessary and sufficient condition for base manifold to be a locally product manifold and proving new conditions to be t…

Semi-Riemannian submersions from real and complex pseudo-hyperbolic spaces

We classify the semi-Riemannian submersions from a pseudo-hyperbolic space onto a Riemannian manifold under the assumption that the fibres are connected and totally geodesic. Also we obtain the classification of the semi-Riemannian submersions from a complex pseudo-hyperbolic space onto a Riemannian manifold under the assumption that the fibres are complex, connected and totally geodesic submanifolds.

Harmonic contact metric structures, and submersions

We study harmonic almost contact structures in the context of contact metric manifolds, and an analysis is carried out when such a manifold fibres over an almost Hermitian manifold, as exemplified by the Boothby-Wang fibration. Two types of almost contact metric warped products are also studied, relating their harmonicity to that of the almost Hermitian structure on the base or fibre.

Optimal symplectic connections on holomorphic submersions

The main result of this paper gives a new construction of extremal Kähler metrics on the total space of certain holomorphic submersions, giving a vast generalisation and unification of results of Hong, Fine and others. The principal new ingredient is a novel geometric partial differential equation on such fibrations, which we call the optimal symplectic connection equation. We begin with a smooth fibration for which all fibres admit a constant scalar curvature Kähler metric. When the fibres admit automorphisms, such metrics are not unique in general, but rather are unique up to the action of t…

Substantial Riemannian Submersions of S(15) with 7- dimensional fibres

In this paper we show that a substantial Riemannian submersion of S(15) with 7- dimensional fibres is congruent to the standard Hopf fibration. As a consequence we prove a slightly weak form of of the Diameter Rigidity theorem for the Cayley plane which is considerably stronger than the very recent Radius Rigidity Theorem of Wilhelm.

一类广泛的接触复黎曼淹没

A Wide Class of Contact-Complex Riemannian Submersions

本文研究全空间为局部共形近拟Sasakian流形的接触复黎曼淹没，确定纤维的Chinea-Gonzalez类与基空间的Gray-Hervella类，并利用O'Neill不变量讨论水平分布可积性与纤维极小性。

On Slant Riemannian Submersions For Cosymplectic Manifolds

In this paper we introduce slant Riemannian submersions from cosymplectic manifolds onto Riemannian manifolds. We obtain some results on slant Riemannian submersions of a cosymplectic manifolds. We also give examples and inequalities between the scalar curvature and squared mean curvature of fibres of such slant submersions according to characteristic vector field is vertical or horizontal.

Riemannian submersions from almost contact metric manifolds

In this paper we obtain the structure equation of a contact-complex Riemannian submersion and give some applications of this equation in the study of almost cosymplectic manifolds with Kaehler fibres.

Totally umbilic null hypersurfaces in generalized Robertson-Walker spaces

We show that there is a correspondence between totally umbilic null hypersurfaces in generalized Robertson-Walker spaces and twisted decompositions of the fibre. This allows us to prove that nullcones are the unique totally umbilic null hypersurfaces in the closed Friedmann Cosmological model. We also apply this kind of ideas to static spaces, in particular to Reissner-Nordström and Schwarzschild exterior spacetimes.

Clairaut 共形淹没

Clairaut Conformal Submersions

本文引入 Riemann 流形间的 Clairaut 共形淹没，刻画其充要条件、纤维几何、曲率、调和性及度量变换，并给出示例与局部模型。

DDVV-type inequality for skew-symmetric matrices and Simons-type inequality for Riemannian submersions

In this paper, we will first derive a DDVV-type optimal inequality for real skew-symmetric matrices, then we apply it to establish a Simons-type integral inequality for Riemannian submersions with totally geodesic fibres and Yang-Mills horizontal distributions. In this way, we show phenomenons of duality between Submanifold geometry and Riemannnian submersion, particularly between second fundamental form of a submanifold and integrability tensor of a Riemannian submersion.

逐点斜黎曼 submersion 沿垂直与水平分布的 Casorati 不等式

Casorati inequalities along vertical and horizontal distributions of pointwise slant riemannian submersions

该研究针对逐点斜黎曼 submersion 的垂直、水平分布建立最优 Casorati 不等式，界定其归一化标量曲率，特殊情形涵盖多种空间形式并给出几何特征，验证了结果的严格性。

保持黎曼淹没及其纤维几何的度量变分

Variations of metric that preserve a Riemannian submersion and geometry of its fibers

本文研究保持黎曼淹没及其纤维诱导度量不变的度量变分，推导纤维第二基本形式变分公式，给出保持全测地、全脐或极小纤维的充要条件，并证明相关泛函临界点处第二变分非负，同时考察截面曲率变分。

Clairaut黎曼扭曲积浸没的几何

Geometry of Clairaut Riemannian warped product submersions

本文引入并研究Clairaut黎曼扭曲积浸没，建立其满足Clairaut条件的充要条件，推导源空间曲率张量显式公式，分析相关几何性质并解答文献[2]推广问题。

Optimal Symplectic Connections and Deformations of Holomorphic Submersions

We give a general construction of extremal Kaehler metrics on the total space of certain holomorphic submersions, extending results of Dervan-Sektnan, Fine, and Hong. We consider submersions whose fibres admit a degeneration to Kaehler manifolds with constant scalar curvature, in a way that is compatible with the fibration structure. Thus we allow fibres that are K-semistable, rather than K-polystable; this is crucial to moduli theory. On these fibrations we phrase a partial differential equation whose solutions, called optimal symplectic connections, represent a canonical choice of a relative…

同构非线性分裂与芬斯勒浸入

Homogeneous nonlinear splittings and Finsler submersions

本文提出了一种基于同构非线性分裂和非线性提升的方法，用于构造不同空间之间的浸入，并给出了新的芬斯勒函数例子。

The Kasparov product on submersions of open manifolds

We study the Kasparov product on (possibly non-compact and incomplete) Riemannian manifolds. Specifically, we show on a submersion of Riemannian manifolds that the tensor sum of a regular vertically elliptic operator on the total space and an elliptic operator on the base space represents the Kasparov product of the corresponding classes in KK-theory. This construction works in general for symmetric operators (i.e. without assuming self-adjointness), and extends known results for submersions with compact fibres. The assumption of regularity for the vertically elliptic operator is not always sa…

基于黎曼淹没和等谱轨道梯度流的特征值过程

Eigenvalue processes in light of Riemannian submersion and gradient flow of isospectral orbits

本文利用黎曼几何语言，证明了多种特征值过程是紧 Lie 群等距作用诱导的黎曼淹没投影结果，统一处理了不同设定下的特征值过程，并据此推导出特征向量过程和一般 $β>0$ 的 $β$ 过程。

Inverse curvature flows in Riemannian warped products

The long-time existence and umbilicity estimates for compact, graphical solutions to expanding curvature flows are deduced in Riemannian warped products of a real interval with a compact fibre. Notably we do not assume the ambient manifold to be rotationally symmetric, nor the radial curvature to converge, nor a lower bound on the ambient sectional curvature. The inverse speeds are given by powers $p\leq 1$ of a curvature function satisfying few common properties.