# Second order elliptic operators on triple junction surfaces

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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 talk about the Morse index on minimal triple junction surfaces.

# A Donaldson-Uhlenbeck-Yau theorem for normal projective varieties

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The correspondence between polystable reflexive sheaves on compact Kaehler manifolds and the existence of suitably singular Hermitian-Einstein metrics can be extended to normal projective varieties that are smooth in codimension two. A particular application is a characterization of those sheaves which saturate the Bogomolov-Gieseker inequality. This talk will present some of the key details of this result, which is joint work with Xuemiao Chen.

# Boundary value problems for first-order elliptic operators with compact and noncompact boundary.

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The index theorem for compact manifolds with boundary, established by Atiyah-Patodi-Singer in the mid-70s, is considered one of the most significant mathematical achievements of the 20th century. An important and curious fact is that local boundary conditions are topologically obstructed for index formulae and non-local boundary conditions lie at the heart of this theorem. Consequently, this has inspired the study of boundary value problems for first-order elliptic differential operators by many different schools, with a class of induced operators adapted to the boundary taking centre stage in formulating and understanding non-local boundary conditions.

That being said, much of this analysis has been confined to the situation when adapted boundary operators can be chosen self-adjoint. Dirac-type operators are the quintessential example. Nevertheless, natural geometric operators such as the Rarita-Schwinger operator on 3/2-spiniors, arising from physics in the study of the so-called Delta baryon, falls outside of this class. Analytically, this requires analysis beyond self-adjoint operators. In recent work with Bär, the compact boundary case is handled for general first-order elliptic operators, using spectral theory to choose adapted boundary operators to be invertible bi-sectorial. The Fourier circle methods present in the self-adjoint analysis are replaced by the bounded holomorphic functional calculus, coupled with pseudo-differential operator theory and semi-group techniques. This allows for a full understanding of the maximal domain of the interior operator as a bounded surjection to a space on the boundary of mixed Sobolev regularity, constructed from spectral projectors associated to the adapted boundary operator. Regularity and Fredholm extensions are also studied.

For the noncompact case, a preliminary trace theorem as well as regularity theory are handed by resorting to the case with compact boundary. This necessitates deforming the coefficients of the interior operator in a compact neighbourhood. Therefore, even for Dirac-type operators, allowing for fully general symbols in the compact boundary case is paramount. Under slightly stronger geometric assumptions near the noncompact boundary (automatic for the compact case) and when the interior operator admits a self-adjoint adapted boundary operator, an upgraded trace theorem mirroring the compact setting is obtained. Importantly, there is no spectral assumptions other than self-adjointness on the adapted boundary operator. This, in particular, means that the spectrum of this operator can be the entire real line. Again, the primarily tool that is used in the analysis is the bounded holomorphic functional calculus.

# Blowup of extremal metrics along submanifolds

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We give conditions under which the blowup of an extremal Kähler manifold along a submanifold of codimension greater than two admits an extremal metric. This generalizes the work of Arezzo-Pacard-Singer, who considered blowups in points. This is a joint work with Gábor Székelyhidi.

# Algebroids for membranes, strings, and particles

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I will introduce G-algebroids — structures related to the symmetries and low-energy effective descriptions of membranes, strings, and particles. I will describe some basic properties of these algebroids and show how they relate to the more standard Lie and Courant algebroids. I will finish by discussing the main classification results and some (spherical) examples. This is a joint work with M. Bugden, O. Hulik, and D. Waldram.

# On the stability of self-similar blow-up for nonlinear wave equations

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One of fundamental importance in studying nonlinear wave equations is the singularity development of the solutions. Within the context of energy supercritical wave equations, a typical way to investigate singularity development is through the self-similar blowup.

In this talk, we will discuss current work in progress toward establishing the asymptotic nonlinear stability of self-similar blowup in the strong-field Skyrme equation and the quadratic wave equation.

# A Sharp Li-Yau gradient bound on Compact Manifolds

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We present a recent result showing that a sharp Li-Yau gradient bound for positive solutions of the heat equation holds for all compact manifolds. However, no sharp Li-Yau bound holds for all noncompact manifolds. This answers an open question by a number of people.

# Finite Element Complexes

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A Hilbert complex is a sequence of Hilbert spaces connected by a sequence of closed densely defined linear operators satisfying the property: the composition of two consecutive maps is zero. The most well-known example is the de Rham complex involving grad, curl, and div operators. A finite element complex is a discretization of a Hilbert complex by replacing infinite dimensional Hilbert spaces by finite dimensional subspaces based on a mesh of the domain. Usually inside each element of the mesh, polynomial spaces are used and suitable degree of freedoms are proposed to glue them to form a conforming subspace. The finite element de Rham complexes are well understood and can be derived from the framework Finite Element Exterior Calculus (FEEC).

In this talk, we will survey the construction of finite element complexes. We present finite element de Rham complex by a geometric decomposition approach. We then generalize the construction to smooth FE de Rham complexes and derive more complexes including the Hessian complex, the elasticity complex, and the divdiv complex in two dimensions by the Bernstein-Gelfand-Gelfand (BGG) construction.

We also present a direct approach for constructing finite element tensors. We construct polynomial complexes and Koszul type complexes, which leads to decompositions of polynomial spaces. We then characterize trace operators using Green’s identity. We construct conforming finite elements for tensor functions with extra constraint.

The constructed finite element complexes will have application in the numerical simulation of the biharmonic equation, the linear elasticity, the general relativity, and in general PDEs in Riemannian geometry etc.

This is a joint work with Xuehai Huang from Shanghai University of Finance and Economics.

The zoom Link to the meeting is

# Positive Scalar Curvature and the Einstein Constraint Equations

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I’ll describe some recent results on the geometry and topology of manifolds with positive scalar curvature (PSC), and the related subject of the constraint equations of general relativity. I’ll cover a number of results which generalize various classics : Schoen-Yau’ 1979 topological obstructions to PSC, Schoen-Yau’s 1979 Positive Mass Theorem, and the Density Theorem of Corvino-Schoen 2005. In dimensions less than 8, these results settle the positive mass with arbitrary ends conjecture made in Schoen-Yau 1988, and the positive mass theorem for initial data sets with boundary (the case without boundary was resolved by Eichmair-Huang-Lee-Schoen 2015, Huang-Lee 2017, Eichmair 2013). Time permitting, I’ll discuss the proof of one of these in more detail. This is based on four papers, two with R.Unger and S.T.Yau, and two with D.Lee and R.Unger.