A remark on Ricci Breathers and Solitons

Eigenvalues and energy functionals with monotonicity formulae under Ricci flow

In this note, we construct families of functionals of the type of $\mathcal{F}$-functional and $\mathcal{W}$-functional of Perelman. We prove that these new functionals are nondecreasing under the Ricci flow. As applications, we give a proof of the theorem that compact steady Ricci breathers must be Ricci-flat. Using these new functionals, we also give a new proof of Perelman's no non-trivial expanding breather theorem. Furthermore, we prove that compact expanding Ricci breathers must be Einstein by a direct method. In this note, we also extend X. Cao's methods of eigenvalues\cite{C} and impro…

A simple proof on the non-existence of shrinking breathers for the Ricci flow

Suppose $M$ is a compact n-dimensional manifold, $n\ge 2$, with a metric $g_{ij}(x,t)$ that evolves by the Ricci flow $\partial_tg_{ij}=-2R_{ij}$ in $M\times (0,T)$. We will give a simple proof of a recent result of Perelman on the non-existence of shrinking breather without using the logarithmic Sobolev inequality.

Monotonicity formulas of eigenvalues and energy functionals along the rescaled List's extended Ricci flow

In this paper, we study monotonicity formulas of eigenvalues and entropies along the rescaled List's extended Ricci flow. We derive some monotonicity formulas of eigenvalues of Laplacian which generalize those of Li in [8] and Cao-Hou-Ling in [3]. Moreover, we also consider monotonicity formulas of $\mathcal{F}_k$-functional which can be seen as a generalized $\mathcal{F}$-functional corresponding with steady Ricci breathers, and $\mathcal{W}_k$-functional which generalizes $\mathcal{W}$-functional corresponding with expanding Ricci breathers.

古老ARF Ricci流上的动态泛函

Dynamical functionals on ancient ARF Ricci flows

该研究在紧致古老ARF Ricci流上引入动态能量泛函，其满足稳态Ricci breather型刚性并为λ-泛函提供上界，还推导了与共轭热流耦合的归一化Ricci流的局部特征值估计。

Eigenvalues and entropies under the harmonic-Ricci flow

In this paper, the author discusses the eigenvalues and entropies under the harmonic-Ricci flow, which is the Ricci flow coupled with the harmonic map flow. We give an alternative proof of results for compact steady and expanding harmonic-Ricci breathers. In the second part, we derive some monotonicity formulas for eigenvalues of Laplacian under the harmonic-Ricci flow. Finally, we obtain the first variation of the shrinker and expanding entropies of the harmonic-Ricci flow.

Ricci flow on Riemannian groupoids

We study the Ricci flow on Riemannian groupoids. We assume that these groupoids are closed and that the space of orbits is compact and connected. We prove the short time existence and uniqueness of the Ricci flow on these groupoids. We also define a F-functional and derive the corresponding results for steady breathers on these spaces.

Remarks on Hamilton's Compactness Theorem for Ricci flow

A fundamental tool in the analysis of Ricci flow is a compactness result of Hamilton in the spirit of the work of Cheeger, Gromov and others. Roughly speaking it allows one to take a sequence of Ricci flows with uniformly bounded curvature and uniformly controlled injectivity radius, and extract a subsequence that converges to a complete limiting Ricci flow. A widely quoted extension of this result allows the curvature to be bounded uniformly only in a local sense. However, in this note we give a counterexample.

Remarks on Gradient Ricci Solitons

In this paper, we study the gradient Ricci soliton equation on a complete Riemannian manifold. We show that under a natural decay condition on the Ricci curvature, the Ricci soliton is Ricci-flat and ALE.

An Elementary Remark on a Lower bound for the $μ$-Invariant of Singularity Models for Ricci Flow

Expository observation on the $μ$-invariant of singularity models for Ricci Flow.

Remarks on curvature behavior at the first singular time of the Ricci flow

In this paper, we study curvature behavior at the first singular time of solution to the Ricci flow on a smooth, compact n-dimensional Riemannian manifold $M$, $\frac{\partial}{\partial t}g_{ij} = -2R_{ij}$ for $t\in [0,T)$. If the flow has uniformly bounded scalar curvature and develops Type I singularities at $T$, using Perelman's $\mathcal{W}$-functional, we show that suitable blow-ups of our evolving metrics converge in the pointed Cheeger-Gromov sense to a Gaussian shrinker. If the flow has uniformly bounded scalar curvature and develops Type II singularities at $T$, we show that suitable…

Remarks of weak-compactness along Kahler Ricci flow

We provide a direct proof of time-slice weak compactness along the Kähler Ricci flow on Fano manifolds.

Remarks on the extension of the Ricci flow

We present two new conditions to extend the Ricci flow on a compact manifold over a finite time, which are improvements of some known extension theorems.

Remark on an inequality for closed hypersurfaces in complete manifolds with nonnegative Ricci curvature

We give a simple proof of a recent result due to Agostiniani, Fogagnolo and Mazzieri.

A remark on degenerate singularity in three dimensional Ricci flow

We show that a rescale limit at any degenerate singularity of Ricci flow in dimension 3 is a steady gradient soliton. In particular, we give a geometric description of type I and type II singularities.

关于 Ricci-Bourguignon 调和孤子的一点注记

A remark of Ricci-Bourguignon harmonic soliton

该研究关注 Ricci-Bourguignon 调和孤子的平凡性，并借助 V-调和映射理论分析 Ricci 调和孤子的性质。

关于非负曲率的凯勒轨道折面的评论

Remarks on Kähler orbifolds of non-negative Ricci curvature

本文证明了凯勒轨道折面的非负曲率版本的 Kobayashi 定理，证明了非负曲率的紧凯勒轨道折面在满足一定条件时是单连通的。

Remarks on shrinking gradient Kähler-Ricci solitons with positive bisectional curvature

In this short note, using an argument by Munteanu and Wang, we provide an alternative proof of the fact first obtained by Lei Ni that shrinking gradient Kähler-Ricci solitons with positive bisectional curvature must be compact.

关于$(α,β)$-切触度量流形中Riemann与Ricci孤立子的注记

Remarks on Riemann and Ricci solitons in $(α,β)$-contact metric manifolds

该研究聚焦满足Ricci对称条件的$(α,β)$-切触度量流形，分析其中近Riemann与近Ricci孤立子在势向量场与结构向量场逐点共线时的性质，补充了相关几何结构的研究内容。

Remarks on scalar curvature of Yamabe solitons

In this paper, we consider the scalar curvature of Yamabe solitons. In particular we show that, with natural conditions and non positive Ricci curvature, any complete Yamabe soliton has constant scalar curvature, namely, it is a Yamabe metric. We also show that the quadratic decay at infinity of the Ricci curvature of a complete non-compact Yamabe soliton has non-negative scalar curvature. A new proof of Kazdan-Warner condition is also presented.

Remarks on non-compact complete Ricci expanding solitons

In this paper, we study gradient Ricci expanding solitons $(X,g)$ satisfying $$ Rc=cg+D^2f, $$ where $Rc$ is the Ricci curvature, $c<0$ is a constant, and $D^2f$ is the Hessian of the potential function $f$ on $X$. We show that for a gradient expanding soliton $(X,g)$ with non-negative Ricci curvature, the scalar curvature $R$ has at least one maximum point on $X$, which is the only minimum point of the potential function $f$. Furthermore, $R>0$ on $X$ unless $(X,g)$ is Ricci flat. We also show that there is exponentially decay for scalar curvature for $ε$-pinched complete non-compact expandin…

Long-time behavior of 3 dimensional Ricci flow -- B: Evolution of the minimal area of simplicial complexes under Ricci flow

In this second part of a series of papers on the long-time behavior of Ricci flows with surgery, we establish a bound on the evolution of the infimal area of simplicial complexes inside a 3-manifold under the Ricci flow. This estimate generalizes an area estimate of Hamilton, which we will recall in the first part of the paper. We remark that in this paper we will mostly be dealing with non-singular Ricci flows. The existence of surgeries will not play an important role.

关于$(\u03b5)$-para Sasakian流形中几乎$\u03b7$-Ricci孤立子的注记

Remarks on almost $η$-Ricci solitons in $(\varepsilon)$-para Sasakian manifolds

本文研究满足特定曲率条件的$(\u03b5)$-para Sasakian流形中几乎$\u03b7$-Ricci孤立子，给出梯度情形下Ricci曲率范数估计与标量曲率表达式，并证明Codazzi型Ricci算子下类空/类时流形对应膨胀/收缩孤立子。

A remark on the Bismut-Ricci form on 2-step nilmanifolds

In this note we observe that on a 2-step nilpotent Lie group equipped with a left-invariant SKT structure the (1,1)-part of the Bismut-Ricci form is seminegative definite. As application we give a simplified proof of the non-existence of invariant SKT static metrics on 2-step nilmanifolds and of the existence of a long time solution to the pluriclosed flow in 2-step nilmanifolds.

Remarks about Synthetic Upper Ricci Bounds for Metric Measure Spaces

We discuss various characterizations of synthetic upper Ricci bounds for metric measure spaces in terms of heat flow, entropy and optimal transport. In particular, we present a characterization in terms of semiconcavity of the entropy along certain Wasserstein geodesics which is stable under convergence of mm-spaces. And we prove that a related characterization is equivalent to an asymptotic lower bound on the growth of the Wasseretein distance between heat flows. For weighted Riemannian manifolds, the crucial result will be a precise uniform two-sided bound for \begin{eqnarray*}\frac{d}{dt}\B…

关于收缩梯度Ricci孤子中极小超曲面的注记

Remarks on minimal hypersurfaces in shrinking gradient Ricci solitons

本文证明在标量曲率满足R≥(n-1)λ的收缩梯度Ricci孤子中，任何紧致双侧光滑稳定极小超曲面的第二基本形式和法向Ricci曲率必须为零，且面积极小超曲面的存在会导致流形分裂。

Remark on a diameter bound for complete manifolds with positive Bakry-Émery Ricci curvature

In this paper, we shall give a new upper diameter estimate for complete Riemannian manifolds in the case that the Bakry-Émery Ricci curvature has a positive lower bound and the norm of the potential function has an upper bound. Our diameter estimate improves previous ones obtained by Wei and Wylie (J. Differential Geom. 83, 377--405, 2009) and Limoncu (Math. Z. 271, 715--722, 2012). As an application, we shall give an upper diameter bound for compact Ricci solitons in terms of the maximum value of the scalar curvature. By using such a diameter bound, we shall provide some new sufficient condit…

关于半正全纯截面曲率与拟负 $k$-Ricci 曲率的注记

Remark on semi-positive holomorphic sectional curvature and quasi-negative $k$-Ricci curvature

本文记录了两个注记：对于紧致Kähler流形，半正全纯截面曲率时MRC纤维化的有理维数等于非真正平坦方向的数量；拟负$k$-Ricci曲率或更一般的拟负混合曲率$C_{a,b}$时，典范丛是丰富的。

A remark on odd dimensional normalized Ricci flow

Let $(M^n,g_0)$ ($n$ odd) be a compact Riemannian manifold with $λ(g_0)>0$, where $λ(g_0)$ is the first eigenvalue of the operator $-4Δ_{g_0}+R(g_0)$, and $R(g_0)$ is the scalar curvature of $(M^n,g_0)$. Assume the maximal solution $g(t)$ to the normalized Ricci flow with initial data $(M^n,g_0)$ satisfies $|R(g(t))| \leq C$ and $\int_M |Rm(g(t))|^{n/2}dμ_t \leq C$ uniformly for a constant $C$. Then we show that the solution sub-converges to a shrinking Ricci soliton. Moreover,when $n=3$, the condition $\int_M |Rm(g(t))|^{n/2}dμ_t \leq C$ can be removed.

The Ricci flow of asymptotically hyperbolic mass and applications

We consider the evolution of the asymptotically hyperbolic mass under the curvature-normalized Ricci flow of asymptotically hyperbolic, conformally compactifiable manifolds. In contrast to asymptotically flat manifolds, for which ADM mass is constant during Ricci flow, we show that the mass of an asymptotically hyperbolic manifold of dimension n>2 decays smoothly to zero exponentially in the flow time. From this, we obtain a no-breathers theorem and a Ricci flow based, modified proof of the scalar curvature rigidity of zero-mass asymptotically hyperbolic manifolds. We argue that the nonconstan…

Remarks on non-compact gradient Ricci solitons

In this paper we show how techniques coming from stochastic analysis, such as stochastic completeness (in the form of the weak maximum principle at infinity), parabolicity and $L^p$-Liouville type results for the weighted Laplacian associated to the potential may be used to obtain triviality, rigidity results, and scalar curvature estimates for gradient Ricci solitons under $L^p$ conditions on the relevant quantities.

A remark on Ricci flow of left invariant metrics

We prove that the Ricci flow equation for left invariant metrics on Lie groups reduces to a first order ordinary differential equation for a map $Q : (-a,a) \to UT$, where $UT$ is the group of upper triangular matrices. We decompose the matrix $R_{ij}$ of Ricci tensor coordinates with respect to an orthonormal frame field $E_{i}$ into a sum $\overset{1}{R}_{ij} + \overset{2}{R}_{ij} + \overset{3}{R}_{ij} + \overset{4}{R}_{ij}$ such that, for any $E_{i'} = U^i_{i'} E_i$ with $||U^i_{i'}|| \in O(n)$, $\oversetα{R}_{i'j'} = U_{i'}^i \oversetα{R}_{ij} U^j_{j'}$. This allows us to specify several c…

A Remark on Regular Points of Ricci Limit Spaces

Let $Y$ be a Gromov-Hausdorff limit of complete Riemannian n-manifolds with Ricci curvature bounded from below. A point in $Y$ is called $k$-regular, if its tangent is unique and is isometric to an $k$-dimensional Euclidean space. By \cite{B5}, there is $k>0$ such that the set of all $k$-regular point $\mathcal{R}_k$ has a full renormalized measure. An open problem is if $\mathcal{R}_l=\emptyset$ for all $l<k$? The main result in this paper asserts that if $\mathcal{R}_1\ne \emptyset$, then $Y$ is a one dimensional topological manifold. Our result improves the Handa's result \cite{Honda} that …

Ricci flow coupled with harmonic map flow

We investigate a new geometric flow which consists of a coupled system of the Ricci flow on a closed manifold M with the harmonic map flow of a map phi from M to some closed target manifold N with a (possibly time-dependent) positive coupling constant alpha. This system can be interpreted as the gradient flow of an energy functional F_alpha which is a modification of Perelman's energy F for the Ricci flow, including the Dirichlet energy for the map phi. Surprisingly, the coupled system may be less singular than the Ricci flow or the harmonic map flow alone. In particular, we can always rule ou…

Some remarks on the Kaehler geometry of LeBrun's Ricci flat metrics on C^2

In this paper we investigate the balanced condition (in the sense of Donaldson) and the existence of an Englis expansion for the LeBrun's metrics on $C^2$. Our first result shows that a LeBrun's metric on $C^2$ is never balanced unless it is the flat metric. The second one shows that an Englis expansion of the Rawnsley's function associated to a LeBrun's metric always exists, while the coefficient $a_3$ of the expansion vanishes if and only if the LeBrun's metric is indeed the flat one.

Remarks on Kahler Ricci Flow

We study some estimates along the Kahler Ricci flow on Fano manifolds. Using these estimates, we show the convergence of Kahler Ricci flow directly if the $α$-invariant of the canonical class is greater than $\frac{n}{n+1}$. Applying these convergence theorems, we can give a flow proof of Calabi conjecture on such Fano manifolds. In particular, the existence of Kahler Einstein metrics on a lot of Fano surfaces can be proved by flow method. Note that this geometric conclusion (based on the same assumption) was established earlier via elliptic method by G. Tian. However, a new proof based on Kah…

Remarks on Entropy Formulae for Linear Heat Equation

In this note, we prove some new entropy formula for linear heat equation on static Riemannian manifold with nonnegative Ricci curvature. The results are analogies of Cao and Hamilton's entropies for Ricci flow coupled with heat-type equations.

Some remarks on Myers theorem for Finsler manifolds

The standard Bonnet-Myers theorem says that if the Ricci scalar of a Riemannian manifold is bounded below by a positive number, then the manifold is compact. Moreover, a bound of its diameter is pointed out. The theorem was extended to Finsler manifolds. In this paper we prove that if a certain condition on the average of the Ricci scalar holds, then the Finsler manifold M is compact if the Ricci scalar is bounded above by the same positive number. An upper bound of the diameter is also found. With no condition on Ricci scalar itself but with a different one on its average, we find that the Fi…

A Lie algebraic approach to Ricci flow invariant curvature conditions and Harnack inequalities

We consider a subset $S$ of the complex Lie algebra $\so(n,\C)$ and the cone $C(S)$ of curvature operators which are nonnegative on $S$. We show that $C(S)$ defines a Ricci flow invariant curvature condition if $S$ is invariant under $\Ad_{\SO(n,\C)}$. The analogue for Kähler curvature operators holds as well. Although the proof is very simple and short it recovers all previously known invariant nonnegativity conditions. As an application we reprove that a compact Kähler manifold with positive orthogonal bisectional curvature evolves to a manifold with positive bisectional curvature and is thu…

The torsion flow on a closed pseudohermitian 3-manifold

In this paper we define the torsion flow, a CR analogue of the Ricci flow. For homogeneous CR manifolds we give explicit solutions to the torsion flow illustrating various kinds of behavior. We also derive monotonicity formulas for CR entropy functionals. As an application, we classify torsion breathers.

Rigidity of four-dimensional Kähler-Ricci solitons

In this article, we investigate four-dimensional gradient shrinking Ricci solitons close to a Kähler model. The first theorem could be considered as a rigidity result for the Kähler-Ricci soliton structure on $\mathbb{S}^2\times \mathbb{R}^2$ (in the sense of Remark 1). Moreover, we show that if the quotient of norm of the self-dual Weyl tensor and scalar curvature is close to that on a Kähler metric in a specific sense, then the gradient Ricci soliton must be either half-conformally flat or locally Kähler.

On a twisted conical Kähler-Ricci flow

In this paper, we discuss diameter bound and Gromov-Hausdorff convergence of a twisted conical Kähler-Ricci flow on the total spaces of some holomorphic submersions. We also observe that, starting from a model conical Kähler metric with possibly unbounded scalar curvature, the conical Kähler-Ricci flow will instantly have bounded scalar curvature for $t>0$, and the bound is of the form $\frac{C}{t}$. Several key results will be obtained by direct arguments on the conical equation without passing to a smooth approximation. In the last section, we present several remarks on a twisted Kähler-Ricc…

关于奇异凯勒-爱因斯坦度量的注记

Remarks on Singular Kähler-Einstein Metrics

本文研究正规复代数簇上奇异凯勒-爱因斯坦度量的两种概念，证明在非塌缩极限情形下较弱概念等价于 Eyssidieux-Guedj-Zeriahi 的较强概念，并推广到定义 RCD 空间的一般奇异凯勒-爱因斯坦空间。

Remarks on homogeneous solitons of the G$_2$-Laplacian flow

We show the existence of expanding solitons of the G$_2$-Laplacian flow on non-solvable Lie groups, and we give the first example of a steady soliton that is not an extremally Ricci pinched G$_2$-structure.

Eigenvalue monotonicity for the Ricci-Hamilton flow, revised version

This is a revised version of our short note [arxiv.math.DG/0403065] where we discuss the monotonicity of the eigen-values of the Laplacian operator to the Ricci-Hamilton flow on a compact or a complete non-compact Riemannian manifold. We show that the eigenvalue of the Lapacian operator on a compact domain associated with the evolving Ricci flow is non-decreasing provided the scalar curvature having a non-negative lower bound and Einstein tensor being not too negative. This result will be useful in the study of blow-up models of the Ricci-Hamilton flow. We remark that using our idea, D.Kokotov…

具有正夹挤Ricci曲率的完备Riemann流形是否是紧的

Is a complete Riemannian manifold with positively pinched Ricci curvature compact

本文针对Hamilton关于欧氏空间中带夹挤第二基本形式的凸超曲面必为紧集的结果，利用作者1998年的拟共形映射结论给出无需额外假设的完整替代证明且可推广，以此纪念Ricci流创立者Hamilton。

关于加权流形中闭超曲面的一个几何不等式的注记

Remark on a geometric inequality for closed hypersurfaces in weighted manifolds

本文在具有非负Bakry-Émery里奇曲率的非紧加权流形中建立闭超曲面的最优几何不等式，刻画了$N=\infty$情形的刚性条件，推广了经典Willmore型不等式。

关于具有特定类型势向量场的Einstein孤立子的注记

Remarks on Einstein solitons with certain types of potential vector field

该研究针对黎曼流形中势向量场为梯度、螺向或共圆型的近Einstein孤立子展开，明确了λ函数的表达式并辅以示例，还推导了Ricci曲率张量满足特定对称条件时的几何性质，丰富了相关微分几何研究。

An Introduction to Conformal Ricci Flow

We introduce a variation of the classical Ricci flow equation that modifies the unit volume constraint of that equation to a scalar curvature constraint. The resulting equations are named the Conformal Ricci Flow Equations because of the role that conformal geometry plays in constraining the scalar curvature. These equations are analogous to the incompressible Navier-Stokes equations of fluid mechanics inasmuch as a conformal pressure arises as a Lagrange multiplier to conformally deform the metric flow so as to maintain the scalar curvature constraint. The equilibrium points are Einstein metr…

关于带有 torsion 的强 $\mathrm{G}_2$ 结构的一些注记

Some remarks on strong $\mathrm{G}_2$-structures with torsion

本文研究了强 $\mathrm{G}_2T$-结构的几何特性，探讨了其与曲率、$S^1$ 作用和几乎 Hermitian 结构的关系，并构造了首个非 Ricci 平坦的强 $\mathrm{G}_2T$-结构例子。

关于带梯度或扭形向量场的近Riemann孤立子的注记

Remarks on almost Riemann solitons with gradient or torse-forming vector field

本文研究黎曼流形中近Riemann孤立子与近Ricci孤立子的关联，利用Bochner公式给出梯度型近Riemann孤立子的λ函数显式表达式，并推导螺管式、扭形势向量场情形下的曲率相关性质。

New curvature flows in complex geometry

This is a survey of some of the recent developments on the geometric and analytic aspects of the Anomaly flow. It is a flow of $(2,2)$-forms on a $3$-fold which was originally motivated by string theory and the need to preserve the conformally balanced property of a Hermitian metric in the absence of a $\partial\bar\partial$-Lemma. It has revealed itself since to be a remarkable higher order extension of the Ricci flow. It has also led to several other curvature flows which may be interesting from the point of view of both non-Kähler geometry and the theory of non-linear partial differential e…