Foreword.- Chapter 1. Definition and Short Time Existence.- Chapter 2. Evolution of Geometric Quantities.- Chapter 3. Monotonicity Formula and Type I Singularities.- Chapter 4. Type II Singularities.- Chapter 5. Conclusions and Research Directions.- Appendix A. Quasilinear Parabolic Equations on Manifolds.- Appendix B. Interior Estimates of Ecker and Huisken.- Appendix C. Hamilton's Maximum Principle for Tensors.- Appendix D. Hamilton's Matrix Li-Yau-Harnack Inequality in Rn.- Appendix E. Abresch and Langer Classification of Homothetically Shrinking Closed Curves.- Appendix F. Important Results without Proof in the Book.- Bibliography.- Index.

Lecture Notes on Mean Curvature Flow

Definition. A (topological) n-manifold M is a Hausdorff topological space with a countable basis of open sets, such that each point of M lies in an open set homeomorphic to R or R+ = {(x1, . . . , xn) ∈ R n : xn ≥ 0}. The boundary ∂M of M is the set of points not having neighbourhoods homeomorphic to R. The set M − ∂M is the interior of M , denoted int(M). If M is compact and ∂M = ∅, then M is closed.

Three-dimensional manifolds

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 out energy concentration of phi a-priori - without any assumptions on the curvature of the target manifold N - by choosing alpha large enough. Moreover, if alpha is bounded away from zero it suffices to bound the curvature of (M,g(t)) to also obtain control of phi and all its derivatives - a result which is clearly not true for alpha = 0. Besides these new phenomena, the flow shares many good properties with the Ricci flow. In particular, we can derive the monotonicity of an entropy functional W_alpha similar to Perelman's Ricci flow entropy W and of so-called reduced volume functionals. We then apply these monotonicity results to rule out non-trivial breathers and geometric collapsing at finite times.

pdf/ricci-flow-coupled-with-harmonic-map-flow-2zvipiq1mh.pdf

Ricci flow coupled with harmonic map flow

We prove precompactness in an orbifold Cheeger–Gromov sense of complete gradient Ricci shrinkers with a lower bound on their entropy and a local integral Riemann bound. We do not need any pointwise curvature assumptions, volume or diameter bounds. In dimension four, under a technical assumption, we can replace the local integral Riemann bound by an upper bound for the Euler characteristic. The proof relies on a Gauss–Bonnet with cutoff argument.

/pdf/a-compactness-theorem-for-complete-ricci-shrinkers-gpujzpa8uq.pdf

A compactness theorem for complete Ricci shrinkers

The main purpose of this paper is to investigate the curvature behavior of four dimensional shrinking gradient Ricci solitons. For such soliton $M$ with bounded scalar curvature $S$, it is shown that the curvature operator $\mathrm{Rm}$ of $M$ satisfies the estimate $|\mathrm{Rm}|\le c\,S$ for some constant $c$. Moreover, the curvature operator $\mathrm{Rm}$ is asymptotically nonnegative at infinity and admits a lower bound $\mathrm{Rm}\geq -c\,\left(\ln r\right)^{-1/4},$ where $r$ is the distance function to a fixed point in $M$. As application, we prove that if the scalar curvature converges to zero at infinity, then the manifold must be asymptotically conical. 
As a separate issue, a diameter upper bound for compact shrinking gradient Ricci solitons of arbitrary dimension is derived in terms of the injectivity radius.

/pdf/geometry-of-shrinking-ricci-solitons-2wexp7ot5y.pdf

Geometry of shrinking Ricci solitons

We define several notions of singular set for Type I Ricci flows and show that they all coincide. In order to do this, we prove that blow-ups around singular points converge to nontrivial gradient shrinking solitons, thus extending work of Naber. As a by-product we conclude that the volume of a finite-volume singular set vanishes at the singular time. We also define a notion of density for Type I Ricci flows and use it to prove a regularity theorem reminiscent of White's partial regularity result for mean curvature flow.

On Type I Singularities in Ricci flow

We dene several notions of singular set for Type I Ricci ows and show that they all coincide. In order to do this, we prove that blow-ups around singular points converge to nontrivial gradient shrinking solitons, thus extending work of Naber [15]. As a byproduct we conclude that the volume of a nite-volume singular set vanishes at the singular time. We also dene a notion of density for Type I Ricci ows and use it to prove a regularity theorem reminiscent of White’s partial regularity result for mean curvature ow [22].

https://homepages.warwick.ac.uk/~maseq/Type1singularities.pdf

On type-I singularities in Ricci flow

/pdf/ricci-flow-coupled-with-harmonic-map-flow-21lx1rbdes.pdf

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On Type I Singularities in Ricci flow

On type-I singularities in Ricci flow

Ricci flow coupled with harmonic map flow

A compactness theorem for complete Ricci shrinkers

Ricci flow coupled with harmonic map flow