In this presentation, we will describe the relationship between various positivity notions in differential geometry and algebraic geometry. We shall also introduce a new concept called "RC-positivity" in differential geometry and use it to characterize uniruled and rationally connected projective manifolds. In particular, we confirm a conjecture of Yau that a compact Kahler manifold with positive holomorphic sectional curvature is projective algebraic and rationally connected.

In this talk I will introduce the computation of the mapping class group of a class of 6-dimensional manifold. This class of manifolds includes 3-dimensional complex complete intersections in complex projective spaces, especially the quintic Calabi-Yau 3-fold. I will compare the result with the classical mapping class group of surfaces. This is a joint work with M.Kreck.

This is a joint work with Lina Chen and Prof. Xiaochun Rong. The Milnor Problem (modified) in the theory of group growth asks whether any finite presented group of vanishing algebraic entropy has at most polynomial growth. It is conjecture by Grigorchuk and Park to be true. We show that a positive answer to the modified Milnor Problem is equivalent to the Nilpotency Conjecture in Riemannian geometry: given n, d>0, there exists a constant /epsilon(n,d)>0 such that if a compact Riemannian n-manifold M satisfies that Ricci curvature >=-(n-1), diameter <=d and volume entropy </epsilon(n,d), then the fundamental group of M has a nilpotent subgroup of finite index. It is our hope that this equivalence will bring geometric tools into the study of Milnor Problem, since by the equivalence progresses made in either problem will shed a light on the other.

Let us consider a sequence of pointed n-manifolds with uniform Ricci curvature lower bound and uniform volume lower bound. By Gromov's pre-compactness theorem, up to a subsequence would converge in Gromov-Hausdorff sense to a metric space (X, d, p). In this talk we will consider the structure of such metric space X and some applications. We will first introduce the results of Cheeger-Colding, and then discuss our recent improvement. This is based on a joint work with Professors Jeff Cheeger and Aaron Naber.

A manifold is said to be of finite topological type if it is homeomorphic to the interior of a compact manifold with boundary. In this talk, I will give a brief introduction to the main results of complete manifolds with nonnegative Ricci curvature and quadratically nonnegatively curved infinity about the finite topological type. This includes some finiteness results under certain conditions of diameter growth (resp. volume growth) and some couterexamples of infinite topology with positive Ricci curvature. This is a joint work with Huihong Zhang.

Let $M$ be a compact Riemannian manifold on which a compact Lie group $G$ acts by isometries. In this talk I will explain how the symmetry induces extra structures in the spectrum of Laplace-type operator, and how to apply symplectic techniques to study the induced equivariant spectrum. This is based on joint works with V. Guillemin and with Y. Qin.

Let M be a closed Riemannian manifold on which the integral ofthe scalar curvature is nonnegative. Suppose a is a symmetric (0,2) tensor field whose dual (1,1) tensor A has n distinct eigenvalues, and tr(A^k) are constants for k = 1, ..., n-1. We show that all the eigenvalues of A are constants,generalizing a theorem of de Almeida and Brito in 1990 to higher dimensions.As a consequence, a closed hypersurface M in S^{n+1} is isoparametric if one takes a above to be the second fundamental form, giving affirmative evidence to Chern's conjecture. This is a joint work with Zizhou Tang and Dongyi Wei.

We study the dynamics of a delayed diffusive hematopoiesis model with two types of Dirichlet boundary conditions. For the model with a zero Dirichlet boundary condition, we establish global stability of the trivial equilibrium under certain conditions, and use the phase plane method to prove the existence and uniqueness of a positive spatially heterogeneous steady state. We further obtain delay-independent as well as delay dependent conditions for the local stability of this steady state. For the model with a non-zero Dirichlet boundary condition, we show that the only positive steady state is a constant solution. Results for the local stability of the constant solution are also provided. By using the delay as a bifurcation parameter, we show that the model has infinite number of Hopf bifurcation values and the global Hopf branches bifurcated from these values are unbounded, which indicates the global existence of periodic solutions.