**Plenary Talk 1: **October 1, 9:00am-10:00am, Click Hall, Alumni Center.

**Title: Finite Element Complexes**

**Abstract**: 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 by the Bernstein-Gelfand-Gelfand (BGG) construction.

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.

**Biography**:

Long Chen is currently a Professor of Mathematics at University of California at Irvine (UCI). He graduated from Nanjing University in 1997, obtained a master's degree from Peking University in 2000, and finished Ph.D from Pennsylvania State University in 2005. His doctoral supervisor is Professor Jinchao Xu. From 2005 to 2007, he worked as a postdoctoral fellow at the University of California, San Diego and the University of Maryland, College Park. Since 2007, he has been working at UCI, and was tenured in 2011, and promoted to full professor in 2015.

Professor Chen's research field is the numerical solution of partial differential equations, especially the design and analysis of finite element methods. In addition, Professor Chen developed the iFEM finite element software package, which provided great convenience for the teaching and research of finite element methods. Professor Chen has published more than 60 academic papers in internationally renowned journals, and serves on the editorial board of several SCI journals. Since his work till now, Professor Chen has been constantly supported by the National Science Foundation.