Organizers: Da Rong Cheng, Greg Galloway and Pengzi Miao

For the Fall 2022 semester, the seminar meets Friday from 4:00 PM to 5:00 PM in Ungar 402 unless otherwise noted.

** Fall 2022 **

**December 2nd (virtual talk, 2:00 PM): ** Thomas Körber (University of Vienna)

*"The Riemannian Penrose inequality for asymptotically flat half-spaces and rigidity"*

Asymptotically flat half-spaces \((M,g)\) are asymptotically flat manifolds with a non-compact boundary. They naturally arise as suitable subsets of initial data for the Einstein field equations. In this talk, I will present a proof of the Riemannian Penrose inequality for asymptotically flat half-spaces with horizon boundary (joint with M. Eichmair) that works in all dimensions up to seven. This inequality gives a sharp bound for the area of the horizon boundary in terms of the half-space mass of \((M,g)\). To prove the inequality, we double \((M, g)\) along its non-compact boundary and smooth the doubled manifold appropriately. To prove rigidity, we use variational methods to show that, if equality holds, the non-compact boundary of \((M,g)\) must be totally geodesic. I will also explain how our techniques can be used to prove rigidity for the Riemannian Penrose inequality for asymptotically flat manifolds.

**November 18th (virtual talk): ** Pengzi Miao (University of Miami)

*"Positive harmonic functions in 3-dimension"*

I will discuss some new properties of positive harmonic functions in dimension three. Applications include families of inequalities relating the surface capacity, Willmore functional, and the mass of asymptotically flat 3-manifolds. A by-product shows additional proofs of the 3-dimensional Riemannian positive mass theorem.

The material of this talk will be a subset of the paper https://arxiv.org/abs/2207.03467.

**November 11th: ** Xiaoxiang Chai (Korea Institute for Advanced Study)

*"Band width estimates in CMC initial data sets and applications"*

Gromov showed that a \(n\) dimensional toroical band with lower scalar curvature bound \(n(n-1)\), the distance of two boundary components of the band is bounded below by \(\pi/n\). There are various generalizations of this band width estimate. We provide a generalization to the spacetime settings. In particular, we study the band width estimate torical band which is also a CMC initial data set. We give a proof using a hypersurface of prescribed null expansion and discuss other proofs. We apply this band width estimates to study the positive mass theorem for asymptotically hyperbolic manifolds with arbitrary ends. This is based joint works of Xueyuan Wan (Chongqing University of Technology).

**November 4th (virtual talk): ** Abraão Mendes (Universidade Federal de Alagoas)

*"Classification of exterior free boundary minimal hypersurfaces"*

In this lecture we aim to present two classification theorems for exterior free boundary minimal hypersurfaces (exterior FBMH for short) in Euclidean space. The first result states that the only exterior stable FBMH with parallel embedded regular ends are the catenoidal hypersurfaces. To achieve this we first prove a Bocher-type result for positive Jacobi functions on regular minimal ends in \(\mathbb{R}^{n+1}\) which, after some calculations, implies the first theorem. The second theorem states that any exterior FBMH \(\Sigma\) with one regular end is a catenoidal hypersurface. Its proof is based on a symmetrization procedure due to R. Schoen. Finally, we give a complete description of the catenoidal hypersurfaces, including the calculation of their indices. This lecture is based on a joint work with L. Mazet.

**October 28th (virtual talk): ** Gaoming Wang (Cornell University)

*"Second order elliptic operators on triple junction surfaces"*

In this talk, we will consider minimal triple junction surfaces, a special class of singular minimal surfaces whose boundaries are identified in a particular manner. Hence, it is quite natural to extend the classical theory of minimal surfaces to minimal triple junction surfaces. Indeed, we can show that the classical PDE theory holds on triple junction surfaces. As a consequence, we can prove a type of Generalized Bernstein Theorem and give the definition of Morse index on minimal triple junction surfaces.

**October 21st (virtual talk): ** Kwok-Kun Kwong (University of Wollongong)

*"Effect of the average scalar curvature on Riemannian manifolds"*

The well-known Bishop-Gromov volume comparison theorem says that if the Ricci curvature is bounded below by \((n-1)k\), then the volume of a metric ball is at most that of the volume of the ball with the same radius in the space form with curvature \(k\). Counterexamples show that the Ricci curvature cannot be replaced by the scalar curvature in the assumption. On the other hand, a Taylor series computation shows that the scalar curvature does tend to decrease the volume of small geodesic balls. In this talk, I will illustrate how the average scalar curvature (together with the Ricci curvature) of a closed manifold affects the average volume of its metric balls of any size. This gives an improvement of the Bishop-Gromov estimate. I will also show its effect on the average total mean curvature of geodesic spheres of radius up to the injectivity radius.

**September 23rd: ** Da Rong Cheng (University of Miami)

*"Existence of Free Boundary Constant Mean Curvature (CMC) Disks"*

Given a surface S in R3, a classical problem is to find disk-type surfaces with prescribed constant mean curvature whose boundary meets S orthogonally. When S is diffeomorphic to a sphere, direct minimization could lead to trivial solutions and hence min-max constructions are needed. Among the earliest such constructions is the work of Struwe, who produced the desired free boundary CMC disks for almost every mean curvature value up to that of the smallest round sphere enclosing S. In a joint work with Xin Zhou (Cornell), we combined Struwe's method with other techniques to obtain an analogous result for CMC 2-spheres in Riemannian 3-spheres and were able to remove the "almost every" restriction in the presence of positive ambient curvature. In this talk, I will report on more recent progress where the ideas in that work are applied back to the free boundary problem to refine and improve Struwe's result.