Martin Kerin
As a means to better understanding manifolds with positive curvature, there has been much recent interest in the study of non-negatively curved manifolds which contain either a point or an open dense set of points at which all 2-planes have positive curvature. We study infinite families of biquotients defined by Eschenburg and Bazaikin from this viewpoint, together with torus quotients of $S^3 \x S^3$.
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On the Ricci curvature of normal metrics on Biquotients
证明了具有有限基本群的闭双商上的任意正规度量都具有正的里奇曲率.
We show there are precisely 15 inhomogeneous biquotients of the form $Sp(3)// Sp(1)^2$ and show that at least 8 of them admit metrics of quasi-positive curvature.
Suppose $ϕ_3:Sp(1)\rightarrow Sp(2)$ denotes the unique irreducible $4$-dimensional representation of $Sp(1) = SU(2)$ and consider the two subgroups $H_1, H_2\subseteq Sp(3)$ with $H_1 = \{\operatorname{diag}(ϕ_3(q_1), q_1): q_1 \in Sp(1)\}$ and $H_2 = \{\operatorname{diag}(ϕ_3(q_2),1):q_2\in Sp(1)\}$. We show that the biquotient $H_1\backslash Sp(3)/H_2$ admits a quasi-positively curved Riemannian metric.
One of the main methods of constructing new spaces with positive or almost positive curvature is the study of biquotients first studied in detail by Eschenburg. We classify orbifold biquotients of the Lie Group $SU(3)$, and construct a new example of a 5-dimensional orbifold with almost positive curvature. Furthermore, we extend the work of Florit and Ziller on the geometric properties of the orbifolds $SU(3)//T^2$.
Cheeger manifolds and the classification of biquotients
一个闭流形称为双商，如果对具有闭子群K和H的紧致李群G，它同构于K \G /H，使得K自由作用在G/H上.双曲率是具有非负截面曲率的黎曼流形的主要例子。我们证明了双商的几个分类结果：（1）我们对所有与双商微分同胚的单连通有理同调球面进行了分类。例如，Gromoll-Meyer奇异球是唯一的奇异球的任何维度，这是一个双商。(2)我们确切地确定哪些Cheeger流形，两个秩一对称空间的连通和，是双元同胚的。例如，CP^2 # CP^2是双商，但CP^4 # HP^2不是。(3)在每个维上，2-连通双连通元的同构类只有1000个。
Examples of biquotients whose tangent bundle is not a biquotient vector bundle
本文研究双商流形切丛何时为双商向量丛，构造了无穷多族切丛不是双商向量丛的双商流形，并给出同一流形两种双商呈现下切丛性质不同的无穷多族例子。
We classify simply connected rationally elliptic manifolds of dimension five and those of dimension six with small Betti numbers from the point of view of their rational cohomology structure. We also prove that a geometrically formal rationally elliptic six dimensional manifold, whose second Betti number is two, is rational cohomology $S^2\times {\mathbb C}P^2$. An infinite family of six-dimensional simply connected biquotients whose second Betti number is three, different from Totaro's biquotients, is considered and it is proved that none of biquotient from this family is geometrically formal…
Symmetries of Eschenburg spaces and the Chern problem
已知的正截面曲率的流形要么是齐性空间，要么是双线性空间，即紧李群被一个群同时作用在左和右上的双线性空间。正曲率齐次度量的全等距群是由K.Shankar确定的。在这里，我们确定的等距群的一些biobiquents由于Escherenburg和Bazaikin。作为一个应用，我们得到，在齐次的情况下，更多的反例陈猜想，其中规定，一个交换子群的基本群的正弯曲的流形是循环的。
Almost positive curvature on the Gromoll-Meyer sphere
Gromoll和Meyer将某个奇异的7-球面$Σ^7$表示为李群$G = Sp(2)$的双商。我们证明了一个2参数家庭的左不变度量$G$的诱导度量$Σ^7$有严格的正截面曲率在所有点以外的余维$\geq 1$，我们明确描述的四个子品种。
We characterise simply-connected biquotients which potentially admit metrics of holonomy G_2. We prove that there are at most three real homotopy types of rationally elliptic such manifolds---all of them being formal. In the course of this examination we classify rationally elliptic homotopy types and characterise 7-dimensional simply-connected biquotients from a rational point of view. Moreover, we also investigate further manifolds of special holonomy, like manifolds of holonomy Spin(7) or Sp(n)\Sp(1) in special situations provided by rational ellipticity or geometric formality.
Metrics of positive Ricci curvature on quotient spaces
本文证明了具有有限基本群的闭双商存在正里奇曲率度量。又设M是一个闭流形，紧李群G以上齐次李群作用在M上，L是G的一个自由作用在M上的闭子群.我们表明，商N：= M/L进行度量的非负Ricci和几乎非负截面曲率。此外，若N有有限个基本群，则N也容许正的里奇曲率度量.特别的例子包括具有有理上同调环的单连通流形的无限族和复和四元数射影空间的积分同调。
We examine several classes of manifolds which have the same cohomology ring as an Eschenburg space (a family of biquotients which is a main source of manifolds with positive curvature). One family are the 3-sphere bundles over CP^2. Another are the circle bundles over a base, which itself is one of the family of CP^1 bundles over CP^2. We classify such manifolds up to diffeomorphism using the Kreck-Stolz invariants. Comparisons of the invariants is then used to find many diffeomorphism of the total space of these bundles with positively curved Eschenburg spaces. The total space of each bundle …
On Closed 6-Manifolds Admitting Riemannian Metrics with Positive Sectional Curvature and Non-Abelian Symmetry
该文研究具有正截面曲率且容许SU(2)或SO(3)等距作用的闭单连通6维黎曼流形，确定其欧拉示性数与已知例子一致，分类相关作用并给出例外层限制。
Topological properties of Eschenburg spaces and 3-Sasakian manifolds
本文研究了7维双曲元diag（z^k1，z^k2，z^k3）\SU（3）/diag（z^l1，z^l2，z^l3）的拓扑性质.这些空间的一个子类带有3-Sasakian度量。作者证明了在这个子族中存在许多同胚但不同胚的3-Sasakian空间。此外，他们还构造了一对3-Sasakian空间，它们是同构的，从而给出了第一个带有两个非等距3-Sasakian度量的流形的例子。这回答了C的一个开放问题。作者声明：by K. Galicki.在这类空间中，作者构造了许多同胚但非同胚的正曲的Schenburg空间对。这类对首先由Kreck-Stolz在Aloff-Wallach空间的特殊子族中构造。
Geometric realizations of curvature models by manifolds with constant scalar curvature
我们证明了任何黎曼曲率模型都可以由具有常数数量曲率的流形几何地实现。我们还证明了任意的伪厄米特曲率模型、仿厄米特曲率模型、超伪厄米特曲率模型和超仿厄米特曲率模型都可以由具有常标量和 *-数量曲率的流形来实现.
In this note, we prove that for a complete noncompact three dimensional Riemannian manifold with bounded sectional curvature, if it has uniformly positive scalar curvature, then there is a uniform lower bound on the injectivity radius.
We establish variational formulas for Ricci upper and lower bounds, as well as a derivative formula for the Ricci curvature. As applications, constant curvature manifolds, Einstein manifolds and Ricci parallel manifolds are identified, respectively, with different integral-differential formulas and semigroup inequalities. Moreover, by using derivative and Hessian formulas for the heat semigroup $P_t$ developed from stochastic analysis, explicit Hessian estimates are derived on Einstein and Ricci parallel manifolds.
The infimum of the Weyl functional is shown to be surprisingly small on many compact 4-manifolds that admit positive-scalar-curvature metrics. Results are also proved that systematically compare the scalar and self-dual Weyl curvatures of certain almost-Kaehler 4-manifolds.
In this work, we construct distance like functions with integral hessian bound on manifolds with small curvature concentration and use it to construct Ricci flows on manifolds with possibly unbounded curvature. As an application, we study the geometric structure of those manifolds without bounded curvature assumption. In particular, we show that manifolds with Ricci lower bound, non-negative scalar curvature, bounded entropy, Ahlfors $n$-regular and small curvature concentration are topologically Euclidean.
Almost-Kähler four manifolds with nonnegative biorthogonal curvature
本文对具有非负双正交曲率的紧致近凯勒四维流形进行了完整分类。
We obtained that any 2-form and any smooth function on 2-manifolds with boundary can be realized as the curvature form and the gaussian curvature function of some Riemmanian metric, respectively.
On the pseudohermitian sectional curvature of a strictly pseudoconvex CR manifold
研究了CR流形的伪厄米截面曲率。
On manifolds with quadratic curvature decay
给出了具有二次曲率衰减的完备非紧流形具有有限拓扑型的条件。特别是，我们发现之间的联系的拓扑结构的流形与二次曲率衰减和一些性质的渐近锥面这样的流形。
It is proved that if an AK2-manifold of dimension greater or equal to 6 is of pointwise constant antiholomorphic sectional curvature, then it is a 6-dimensional manifold of constant negative sectional curvature or a Kähler manifold of constant holomorphic sectional curvature.
We prove a mixed curvature analogue of Gromov's almost flat manifolds theorem for upper sectional and lower Bakry-Emery Ricci curvature bounds.
In this paper paraSasakian manifolds with a constant paraholomorphic section curvature are considered.
Manifolds with A Lower Ricci Curvature Bound
本文综述了具有里奇曲率下界的流形的结构。
In a previous paper, we constructed complete manifolds of positive Ricci curvature with quadratically asymptotically nonnegative curvature and infinite topological type but dimension $\ge 6$. The purpose of the present paper is to use a different way to exhibit a family of complete $I$-dimensinal ($I\ge5$) Riemannian manifolds of positive Ricci curvature, quadratically asymptotically nonnegative sectional curvature, and certain infinite Betti number $b_j$ ($2\le j\le I-2$).
In this paper, we study the interplay of geometry and positive scalar curvature on a complete, non-compact manifold with non-negative Ricci curvature. In three-dimensional manifold, we prove a minimal volume growth, an estimate of integral of scalar curvature and width. In higher dimensional manifold, we obtain a volume growth with a stronger condition.
This note is devoted to study the implications of nonpositive isotropic curvature and negative Ricci curvature for Einstein $4-$Manifolds.
In this paper, we study the integral curvatures of Finsler manifolds and prove several Myers type theorems.
We prove a sharp Sobolev inequality on manifolds with nonnegative Ricci curvature. Moreover, we prove a Michael-Simon inequality for submanifolds in manifolds with nonnegative sectional curvature. Both inequalities depend on the asymptotic volume ratio of the ambient manifold.
An identity of conformal-projective curvature tensor of a statistical manifold is studied in this paper. The relation between the constancy of curvature and conformal-projective flatness of statistical manifolds is also discussed.
In this note we classify compact 4-manifolds with harmonic Weyl tensor and nonnegative biorthogonal curvature
Riemannian metrics of positive Ricci curvature were constructed on certain moment-angle manifolds.
Manifolds with Pointwise Ricci Pinched Curvature
本文证明了具有任意点态压缩里奇曲率张量的黎曼流形的一个紧性结果。
Complete k-curvature homogeneous pseudo-Riemannian manifolds 0-modeled on an indecomposible symmetric space
对于k至少为2，我们展示了完整的k-曲率齐性中性签名伪黎曼流形，它不是局部仿射齐性的（因此不是局部齐性的）。这些流形的曲率张量是仿照一个不可分解的对称空间。所有的局部标量外尔曲率不变量的这些流形消失。
In this paper we prove a compactness theorem for constant mean curvature surfaces with area and genus bound in three manifold with positive Ricci curvature. As an application, we give a lower bound of first eigenvalue of constant mean curvature surfaces in three manifold with positive Ricci curvature.
Complete k-curvature homogeneous pseudo-Riemannian manifolds
对于任何k至少为2，我们展示了完整的k-曲率齐性中性签名伪黎曼流形不是k+1-仿射曲率齐性，因此不是局部齐性。所有的局部标量Weyl不变量的这些流形消失。这些流形是Ricci flat，Osserman和Ivanov-Petrova。
Compact four-manifolds with pinched self-dual Weyl curvature
本文研究了具有调和自双曲Weyl曲率和挤压条件的紧致四流形的性质。
We will show that a statistical manifold $(M, g, \nabla)$ has a constant curvature if and only if it is a projectively flat conjugate symmetric manifold, that is, the affine connection $\nabla$ is projectively flat and the curvatures satisfies $R=R^*$, where $R^*$ is the curvature of the dual connection $\nabla^*$. Moreover, we will show that properly convex structures on a projectively flat compact manifold induces constant curvature $-1$ statistical structures and vice versa.
We draw elliptic regularity results for 4-manifolds with an elliptic system, without Sobolev constant control. Direct use of analysis is circumvented; the results come mainly through geometric and topological arguments. In contrast to our previous paper, which worked predominantly on the scale of the curvature radius, the results here provide curvature controls on a fixed scale.
On stability and scalar curvature rigidity of quaternion-Kähler manifolds
本文证明负标量曲率的四元数-凯勒流形作为爱因斯坦流形稳定且标量曲率刚性，并推广到特殊和乐情形，同时构造正标量曲率的半稳定但非刚性的反例。
We show any Weyl curvature model can be geometrically realized by a Weyl manifold
We study curvature identities on contact metric manifolds on the geometry of the corresponding almost Käehler cones, and we provide applications of the derived curvature identities.
Patterson discussed the curvature identities on Riemannian manifolds in [14], and a curvature identity for any 6-dimensional Riemannian manifold was independently derived from the Chern-Gauss-Bonnet Theorem [8]. In this paper, we provide the explicit formulae of Patterson's curvature identity that holds on 5-dimensional and 6-dimensional Einstein manifolds. We confirm that the curvature identities on the Einstein manifold from the previous work [8] are the same as the curvature identities deduced from Patterson's result. We also provide examples that support the theorems.
We derive a curvature identity that holds on any 6-dimensional Riemannian manifold, from the Chern-Gauss-Bonnet theorem for a 6-dimensional closed Riemannian manifold. We also introduce some applications of this curvature identity.
A closed symplectic four-manifold has almost Kaehler metrics of negative scalar curvature
证明了每个闭辛四维流形都有相容的几乎Kaehler度量的负数量曲率.
Prescribed Chern scalar curvature on complete noncompact Hermitian manifolds with nonpositive curvatures
本文研究了在非正曲率的完整非紧赫尔米特流形上规定陈标量曲率的问题，得出了存在常数负陈标量曲率度量的充分条件。
Quaternionic-Kähler manifolds with nonnegative sectional curvature
本文证明具有正数量曲率和非负截面曲率的紧四元Kähler流形等距于对称空间，推广了Berger的经典定理。
In this paper, by introducing a notion of local quasi holomorphic frame, we obtain a curvature formula for almost Hermitian manifolds which is similar to that of Hermitian manifolds. Moreover, as applications of the curvature formula, we extend a result of H.S. Wu and a result of F. Zheng to almost Hermitian manifolds.
For the Bach-flat closed manifold with positive scalar curvature, we prove a rigidity result under a given inequality involving the Weyl curvature and the traceless Ricci curvature. Moveover, under an inequality involving $L^{\frac{n}{2}}$-norm of the Weyl curvature, the traceless Ricci curvature and the Yamabe invariant, we also provide a similar rigidity result. As an application, we obtain some rigidity results on 4-dimensional manifolds.
We first provide an alternative proof of the classical Weitzneböck formula for Einstein four-manifolds using Berger curvature decomposition, motivated by which we establish a unified framework for a Weitzenböck formula for a large class of canonical metrics on four-manifolds (or a Weitzenböck formula for "Einstein metrics" on four-dimensional smooth metric measure spaces). As applications, we classify Einstein four-manifolds of half two-nonnegative curvature operator, which in some sense provides a characterization of Kähler-Einstein metrics with positive scalar curvature on four-manifolds, we…
We present several problems and results relating the scalar curvatures of manifolds with mean curvatures of their boundaries
This is a survey of recent results on manifolds with positive curvature from a series of lecture given in Guanajuato, Mexico in 2010. It also contains some hitsorical comments.
Kahler manifolds with quasi-constant holomorphic curvature
本文的目的是对具有拟常全纯截面曲率的紧致Kahler流形进行分类。
We extend results of Llarull and Goette-Semmelmann to manifolds with boundary.
There is a conjecture that a complete Riemannian 3-manifold with bounded sectional curvature, and pointwise pinched nonnegative Ricci curvature, must be flat or compact. We show that this is true when the negative part (if any) of the sectional curvature decays quadratically.
In this note we extend a recent result of S. Brendle [3] to Riemannian manifolds with densities and nonnegative Bakry-Émery Ricci curvature.
Hermitian manifolds of pointwise constant antiholomorphic sectional curvatures
在大于4维情形下，证明了如果Hermitian非Kaehler流形具有逐点常反全纯截面曲率，则它也具有常截面曲率.
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