Anand Dessai
We show that the indices of certain twisted Dirac operators vanish on a $Spin$-manifold $M$ of positive sectional curvature if the symmetry rank of $M$ is $\geq 2$ or if the symmetry rank is one and $M$ is two connected. We also give examples of simply connected manifolds of positive Ricci curvature which do not admit a metric of positive sectional curvature and positive symmetry rank.
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It is shown that the variational derivative of the integral of Branson's Q-curvature is the ambient obstruction tensor of Fefferman-Graham. A classification of irreducible conformally invariant tensors modulo quadratic and higher degree terms in curvature is established.
Some obstructions to positive scalar curvature on a noncompact manifold
本文研究非紧流形上存在完备正数量曲率度量的障碍，并分别对有限体积与无限体积情形给出判定条件。
We provide integral curvature bounds for compact Riemannian manifolds that allow isometric immersions into a Euclidean space with low codimension in terms of the Betti numbers.
Torsion Obstructions to Positive Scalar Curvature
本文研究由闭光滑流形基本群整同调中的挠类引起的正数量曲率度量存在性的阻碍，并构造了不承认正数量曲率度量但其笛卡尔积承认此类度量的流形新例子。
Motivated to study the geometry of the exotic spheres constructed in [5], we derive a necessary condition for non-negative sectional curvature in certain total spaces of Riemannian submersions with totally geodesic fibers. In particular, we prove that the bundles in [5] and [1] have sections of negative curvature.
Positive Scalar Curvature Obstructions via Singular Dimension Descent
本文发展了Schoen-Yau型奇异维度下降法，用于任意维度的正数量曲率障碍研究，证明了可放大流形上的正数量曲率障碍，并建立了相应的立方宽度不等式和双系统估计。
We establish new obstruction results to the existence of Riemannian metrics on tori satisfying mixed bounds on both their sectional and Ricci curvatures. More precisely, from Lohkamp's theorem, every torus of dimension at least three admits Riemannian metrics with negative Ricci curvature. We show that the sectional curvature of these metrics cannot be bounded from above by an arbitrarily small positive constant. In particular, if the Ricci curvature of a Riemannian torus is negative, bounded away from zero, then there exist some planar directions in this torus where the sectional curvature is…
K. Grove, L. Verdiani, B. Wilking and W. Ziller gave the first examples of cohomogeneity one manifolds which do not carry invariant metrics with non-negative sectional curvatures. In this paper we generalize their results to a larger family. We also classified all class one representations for a pair (G;H) with G/H some sphere, which are used to construct the examples.
The classical theory of $G$-structures, which include almost-complex structures, explains the relationship between the curvature of compatible connections and integrability. This note is an effort to understand how the curvature of Riemannian metrics can obstruct the integrability of almost-complex structures. It is shown that certain special complex structures cannot coexist with non-flat constant curvature metrics, and a formal variational realization of these structures is provided. The approach followed here is direct, meaning that it bypasses the classical theory. The idea is to find obst…
We study topological obstructions to the existence of a Riemannian metric on manifolds with boundary such that the scalar curvature is non-negative and the boundary is mean convex. We construct many compact manifolds with boundary which admit no Riemannian metric with non-negative scalar curvature and mean convex boundary. For example, we show that the manifold $(T^{n-2}\times Σ)\# N$, where $Σ$ is a compact, connected and orientable surface which is not a disk or a cylinder and $N$ is a closed $n$-dimensional manifold, does not admit a metric of non-negative scalar curvature and mean convex b…
In contrast to the homogeneous case, we show that there are compact cohomogeneity one manifolds, that do not support invariant metrics of non-negative sectional curvature. In fact we exhibit infinite families of such manifolds including the exotic Kervaire spheres. Such examples exist for any codimension of the singular orbits except for the case where both are equal to two, where existence of non-negatively curved metrics is known.
We prove a Pohozaev type identity for non-linear eigenvalue equations of the Dirac operator on Riemannian spin manifolds with boundary. As an application, we obtain that the mean curvature H of a conformal immersion S^{n}-> R^{n+1} satisfies $\int \partial_X H=0$ where X is a conformal vector field on S^{n} and where the integration is carried out with respect to the Euclidean volume measure of the image.<BR> This identity is analogous to the Kazdan-Warner obstruction that appears in the problem of prescribing the scalar curvature on S^{n} inside the standard conformal class.
We provide a quantitative obstruction to collapsing surfaces of genus at least 2 under a lower curvature bound and an upper diameter bound. Keywords: curvature; diameter; volume; filling radius; systole; Gromov-Hausdorff distance
Partial Scalar Curvatures and Topological Obstructions for Submanifolds
该研究引入介于最小Ricci曲率与归一化标量曲率之间的内蕴曲率ρₖ，推导空间形式中子流形的相关不等式与基于Betti数的拓扑障碍，并构造出第一Betti数任意大的𝕊⁶中三维极小Wintgen理想子流形验证k=n时障碍不成立。
We find new obstructions to the existence of complete Riemannian metric of nonnegative sectional curvature on manifolds with infinite fundamental groups. In particular, we construct many examples of vector bundles whose total spaces admit no nonnegatively curved metric.
A Gauss-Bonnet-Chern type obstruction for Killing vector fields on Lorentzian manifolds
本文提出了一种新的曲率障碍，利用类时（零）截面曲率刻画偶数维（奇数维）Lorentzian 流形上 Killing 或同位向量场的存在性，并将紧致情形下的 Gauss-Bonnet-Chern 障碍推广至非零类时截面曲率。
We give a geometric obstruction to the non-negativity of the sectional curvature in the total spaces of certain Riemannian submersions with totally geodesic fibers; applications of this obstruction to several examples are given.
Applying a local Gauss-Bonnet formula for closed subanalytic sets to the complex analytic case, we obtain characterizations of the Euler obstruction of a complex analytic germ in terms of the Lipschitz-Killing curvatures and the Chern forms of its regular part. We also prove analogous results for the global Euler obstruction. As a corollary, we give a positive answer to a question of Fu on the Euler obstruction and the Gauss-Bonnet measure.
We first show that existence results due to Kazdan-Warner and Cruz-Vitório can be extended to the category of manifolds with an isolated conical singularity. More precisely, we check that, under suitable conditions on the link manifold, any bounded and smooth function which is negative somewhere is the scalar curvature of some conical metric (with the boundary being minimal whenever it is non-empty). By way of comparison, we complement this analysis by indicating how index theory, as developed by Albin-Gell-Redman, may be used to transfer to this conical setting some of the classical obstructi…
Existence and obstructions for the curvature on compact manifolds with boundary
本文研究带边紧流形上指定曲率函数的存在性问题，证明Gauss-Bonnet符号条件是共形等价度量实现测地曲率或高斯曲率的充要条件，并给出依赖Euler示性数的存在性与不存在性新结果。
Obstructions to prescribed Q-curvature of complete conformal metrics on $\mathbb{R}^n$
本文研究了 $\mathbb{R}^n$ 上完备共形度量的预定 Q-曲率问题，给出了一个 Bonnet-Mayer 型障碍定理，并揭示了预定函数衰减速率带来的其他障碍。
We extend the notions of CR GJMS operators and Q-curvature to the case of partially integrable CR structures. The total integral of the CR Q-curvature turns out to be a global invariant of compact nondegenerate partially integrable CR manifolds equipped with an orientation of the bundle of contact forms, which is nontrivial in dimension at least five. It is shown that its variation is given by the curvature-type quantity called the CR obstruction tensor, which is introduced in the author's previous work. Moreover, we consider the linearized CR obstruction operator. Based on a scattering-theore…
Block and Weinberger show that an arithmetic manifold can be endowed with a positive scalar curvature metric if and only if its $\rationals$-rank exceeds 2. We show in this article that these metrics are never in the same coarse class as the natural metric inherited from the base Lie group. Furthering the coarse $C^\ast$-algebraic methods of Roe, we find a nonzero Dirac obstruction in the $K$-theory of a particular operator algebra which encodes information about the quasi-isometry type of the manifold as well as its local geometry.
We prove integral curvature bounds in terms of the Betti numbers for compact submanifolds of the Euclidean space with low codimension. As an application, we obtain topological obstructions for $δ$-pinched immersions. Furthermore, we obtain intrinsic obstructions for minimal submanifolds in spheres with pinched second fundamental form.
We establish a link between rational homotopy theory and the problem which vector bundles admit complete Riemannian metric of nonnegative sectional curvature. As an application, we show for a large class of simply-connected nonnegatively curved manifolds that, if C lies in the class and T is a torus of positive dimension, then "most" vector bundles over the product of C and T admit no complete nonnegatively curved metric.
Lectures on Immersions with Controlled Curvatures
探讨黎曼流形间具“小”曲率浸入的构造方法，并指出此类浸入存在的障碍。
Twisted $S^1$ stability and positive scalar curvature obstruction on fiber bundles
本文研究纤维丛上的正标量曲率障碍，在纤维不可压缩或同伦非平凡条件下证明多种纤维丛无 PSC 度量，并给出 $S^1$ 丛的 PSC 稳定性等价条件，部分回答 Gromov 问题并推广前人结果。
The scalar curvature in wedge spaces: existence and obstructions
本文研究具有单一奇异层的楔形空间中不完备楔形度量的数量曲率，利用 Yamabe、椭圆和指标理论，在正性假设下给出此类度量的存在性与阻碍结果，并推广了先前关于孤立锥奇点的工作。
Examples of almost-positively and quasi-positively curved spaces of the form M=H((G,h)xF) were discovered recently. Here, h is a left-invariant metric on a compact Lie group G, F is a compact Riemannian manifold on which the subgroup H of G acts isometrically on the left, and M is the orbit space of the diagonal left action of H on (G,h)xF with the induced Riemannian submersion metric. We prove that no new examples of strictly positive sectional curvature exist in this class of metrics. This result generalizes the case F={point} proven by Geroch.
PSC Kähler cones of minimal Kähler surfaces and a birational obstruction
本文刻画了极小紧Kähler曲面的正标量曲率Kähler锥，并证明在双有理态射到斜率不稳定规则曲面时存在无正标量曲率度量的Kähler类，给出双有理障碍。
For a manifold with an affine connection, we prove formulas which infinitesimally quantify the gap in a certain naturally defined open geodesic quadrilateral associated to a pair of tangent vectors $u$, $v$ at a point of the manifold. We show that the 1st order infinitesimal obstruction to the quadrilateral to close is always zero, the 2nd order infinitesimal obstruction to the quadrilateral to close is $-T(u,v)$ where $T$ is the torsion tensor of the connection, and if $T = 0$ then the 3rd order infinitesimal obstruction to the quadrilateral to close is $(1/2)R(u,v)(u+v)$ in terms of the curv…
Resolution of compact Einstein orbifolds in general dimensions
研究高维紧致爱因斯坦流形非塌缩序列的Gromov-Hausdorff极限为紧致爱因斯坦orbifold，证明其存在有限奇点，并给出超几何orbifold的负障碍物，简化了Ozuch四维工作中的技术难点。
We establish fundamental results for a parabolic flow of Riemannian metrics introduced by Bahuaud-Helliwell in arXiv:1010:4287v1 which is based on the Fefferman-Graham ambient obstruction tensor. First, we obtain local $L^2$ smoothing estimates for the curvature tensor and use them to prove pointwise smoothing estimates for the curvature tensor. We use the pointwise smoothing estimates to show that the curvature must blow up for a finite time singular solution. We also use the pointwise smoothing estimates to prove a compactness theorem for a sequence of solutions with bounded $C^0$ curvature …
Kazdan-Warner obstructions for a 4$th-$order boundary problem
本文研究通过共形变换设定非恒定内部Q曲率和边界T曲率的四阶边值问题，利用变分方法和共形向量场获得非平凡积分障碍。
Relative aspherical conjecture and higher codimensional obstruction to positive scalar curvature
研究相对非球面猜想，提出推广模型并引入球面宽度几何量，证明3-流形在余维3和4下对正数量曲率存在的障碍，扩展了维度5以内的非球面猜想。
We show that there are topological obstructions for a noncompact manifold to admit a Riemannian metric with quadratic curvature decay and a volume growth which is slower than that of Euclidean space of the same dimension.
Apart from spheres and an infinite family of manifolds in dimension seven, Bazaikin spaces are the only known examples of simply connected Riemannian manifolds with positive sectional curvature in odd dimensions. We consider positively curved Riemannian manifolds whose universal covers have the same cohomology as Bazaikin spaces and prove structural results for the fundamental group in the presence of torus symmetry
We examine homogeneous solitons of the ambient obstruction flow and, in particular, prove that any compact ambient obstruction soliton with constant scalar curvature is trivial. Focusing on dimension 4, we show that any homogeneous gradient Bach soliton that is steady must be Bach flat, and that the only non-Bach-flat shrinking gradient solitons are product metrics on $\mathbb{R}^2\times S^2$ and $\mathbb{R}^2 \times H^2$. We also construct a non-Bach-flat expanding homogeneous gradient Bach soliton. We also establish a number of results for solitons to the geometric flow by a general tensor $…
This is a survey of the current state of the question "Which closed connected manifolds of dimension $n\ge 5$ admit Riemannian metrics whose scalar curvature function is everywhere positive?" The introduction gives a brief overview of these results, while the body of the paper discusses the methods used in the proofs of these results. We mention the two flavors of topological obstructions to the existence of \pscm s: one is a consequence of the Weizenböck formula for the Dirac operator, the other is obtained by considering stable minimal hypersurfaces. We talk about geometric constructions of …
We analyze the obstruction to metrics of positive scalar curvature within a given bounded distortion class of metrics. This obstruction lives in a non-Hausdorff cohomology group Poincare dual to the uniformly finite homology studied by Block and Weinberger. One of the applications is a converse to their theorem on infinite connected sums, giving a complete picture of when such manifolds have metrics of positive scalar curvature.
Obstructions to distinguished Riemannian metrics via Lorentzian geometry
本研究借助洛伦兹几何的对偶框架，通过平面波极限得到特殊黎曼度量的局部必要条件，并利用Wick旋转排除了一类偏离常曲率的紧致黎曼流形的存在性，为黎曼度量的曲率障碍研究提供了新方法。
We study algebraic conditions on a group G under which every properly discontinuous, isometric G-action on a Hadamard manifold has a G-invariant Busemann function. For such G we prove the following structure theorem: every open complete nonpositively curved Riemannian K(G,1) manifold that is homotopy equivalent to a finite complex of codimension >2 is an open regular neighborhood of a subcomplex of the same codimension. In this setting we show that each tangential homotopy type contains infinitely many open K(G,1) manifolds that admit no complete nonpositively curved metric even though their u…
On the Curvature and Topology of Compact Stationary Spacetimes
本文基于 Petersen 与 Wink 2021 年的结果，研究紧致洛伦兹流形在具有单位长度类时 Killing 向量场时其曲率与拓扑所受到的阻碍。
We discuss the cobordism type of spin manifolds with nonnegative sectional curvature. We show that in each dimension $4k \geq 12$, there are infinitely many cobordism types of simply connected and nonnegatively curved spin manifolds. Moreover, we raise and analyze a question about possible cobordism obstructions to nonnegative curvature.
A Pontryagin class obstruction for purely electric and purely magnetic Weyl curvature tensors
研究纯电或纯磁韦伊尔曲率张量的洛伦兹度量存在性，通过庞特里亚金形式的乘积 vanish 推导 cohomological 障碍，关联到 Petrov 子类型和非退化脐超曲面的分类。
When can minimal hypersurfaces be connected by mean curvature flow?
本文从莫尔斯理论视角研究极小超曲面之间是否存在连接平均曲率流，并通过构造例子揭示了除已知条件外还存在拓扑与变分层面的额外阻碍。
In this paper we obtain rigidity results and obstructions on the topology at infinity of translating solitons of the mean curvature flow in the Euclidean space. Our approach relies on the theory of f-minimal hypersurfaces.
Generalized Ueda Obstruction Classes and Applications to Non-semi-positivity of Line Bundles
引入沿复流形闭解析子簇的线束广义第一Ueda障碍类，允许奇异子簇和非平坦限制，通过Dolbeault分解和Chern曲率，类非零阻碍半正曲率光滑Hermite度量的存在，统一经典例子并给出新的nef非半正线束例子。
We prove that polarised manifolds that admit a constant scalar curvature Kähler (cscK) metric satisfy a condition we call slope semistability. That is, we define the slope $μ$ for a projective manifold and for each of its subschemes, and show that if $X$ is cscK then $μ(Z)\leμ(X)$ for all subschemes $Z$. This gives many examples of manifolds with Kähler classes which do not admit cscK metrics, such as del Pezzo surfaces and projective bundles. If $\PP(E)\to B$ is a projective bundle which admits a cscK metric in a rational Kähler class with sufficiently small fibres, then $E$ is a slope semist…
We give new examples of closed smooth 4-manifolds which support singular metrics of nonpositive curvature, but no smooth ones, thereby answering affirmatively a question of Gromov. The obstruction comes from patterns of incompressible 2-tori sufficiently complicated to force branching of geodesics for nonpositively curved metrics.
In this paper, we consider an obstruction to asymptotic Chow-semistability of a polarized Kaehler algebraic manifold. Even when a linear algebraic group of positive dimension acts nontrivially and holomorphically on a polarized Kaehler algebraic manifold with constant scalar curvature, the vanishing of the obstruction allows us to generalize Donaldson's construction of approximate solutions for equations of balanced metrics.
Let $M^n$ be a compact K$\ddot{a}$hler manifold with almost nonnegative Ricci curvature and nonzero first Betti number. We show that the holomorphic Euler number of $M^n$ vanishes, which gives a new obstruction for compact complex manifolds admitting K$\ddot{a}$hler metrics with almost nonnegative Ricci curvature. A crucial step in the proof is to show a vanishing theorem of Dolbeault-Morse-Novikov cohomology.
The metric extension problem under positive and negative curvature conditions
本文研究正负曲率条件下的度量延拓问题，证明了紧流形上边界度量的延拓定理，并应用于球面的严格凸等距嵌入。
In the present article we obtain classification results and topological obstructions for the existence of translating solitons of the mean curvature flow.
Linking at Infinity and Scalar Curvature Decay on Non-Compact Manifolds
研究非紧致正标量曲率流形上曲率衰减与无穷远处拓扑结构的关系，证明拓扑链接迫使弱有界几何流形上标量曲率多项式衰减，并发展局部于非紧致流形端点的障碍理论。
We determine the local equivalence class of the Seiberg-Witten Floer stable homotopy type of a spin rational homology 3-sphere $Y$ embedded into a spin rational homology $S^{1} \times S^{3}$ with a positive scalar curvature metric so that $Y$ generates the third homology. The main tool of the proof is a relative Bauer-Furuta-type invariant on a periodic-end 4-manifold. As a consequence, we give obstructions to positive scalar curvature metrics on spin rational homology $S^{1} \times S^{3}$, typically described as the coincidence of various Frøyshov-type invariants. This coincidence also yields…
Positive scalar curvature and an equivariant Callias-type index theorem for proper actions
本文研究局部紧群适当作用流形上具有一致正标量曲率的完备不变度量存在性，利用等变 Callias 型指标定理给出障碍，并在连通李群情形下推广 Lawson-Yau 构造消除障碍。
In this short note we discuss some recent results about two-positive Ricci curvature and their applications to positive Einstein curvature.
We study Einstein metrics on smooth compact 4-manifolds with an edge-cone singularity of specified cone angle along an embedded 2-manifold. To do so, we first derive modified versions of the Gauss-Bonnet and signature theorems for arbitrary Riemannian 4-manifolds with edge-cone singularities, and then show that these yield non-trivial obstructions in the Einstein case. We then use these integral formulae to obtain interesting information regarding gravitational instantons which arise as limits of such edge-cone manifolds.
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