In this talk, we will first survey some known result about curvature behavior at the first finite-singularity time under the Ricci flow. Then we will discuss some recent development in this direction and their applications.

Symplectic harmonic theory was initiated by Ehresmann and Libermann in 1940's, and was rediscoverd by Brylinski in late 1980's. More recently, Bahramgiri showed in his MIT thesis that symplectic harmonic representatives of Thom classes exhibited some interesting global feature of symplectic geometry. In this talk, we discuss a new approach to symplectic Harmonic theory via geometric measure theory. The new method allows us to establish a fundamental property on symplectic harmonic forms, which is a non-trivial generalization of Bahramgiri's result, and enables us to provide a complete solution to an open question asked by V. Guillemin concerning symplectic harmonic representatives of Thom classes.  This talk is based on a very recent work of the speaker.

There does not exist closed manifold along the line of projective planes
above the dimension of octonions due to the inexistence of hopf invariant
1 map in higher dimensions. I investigated the existence dimension of such
manifold in the rational sense, such that the rational cohomology is rank
one in dimension 0, 2k and 4k and is zero otherwise. Applying rational
surgery, the problem can be reduced to finding possible Pontryagin classes
satisfying the Hirzebruch signature formula and a set of congruence relations
determined by the Riemann-Roch integrality conditions, which is eventually
equivalent to solving a system of Diophantine equations. After a negative
answer in dimension 24, the first existence dimension of such manifold is 32.

What is the "simplest" knot in a given three-manifold Y?
We know that the answer is the unknot when Y=S^3, as the unknot
happens to be the only knot in the three-sphere with the smallest
genus (=0). In this talk, we will discuss the more general notion of
the rational genus of knots. In particular, we will show that the
simple knots are really the "simplest" knots in the lens spaces in
the sense of being a genus minimizer in its homology class. This is
a joint work with Yi Ni.

In this talk, we study  the Gromov-Hausdorff convergence of  Ricci-flat metrics   under degenerations of Calabi-Yau manifolds. More precisely, for a family of polarized Calabi-Yau manifolds degenerating to a singular Calabi-Yau variety,  we prove that  the Gromov-Hausdorff limit of Ricci-flat kahler metrics on them is unique.