We study curvature estimates of Ricci flow on complete 3-dim manifolds
without bounded curvature assumptions. Especially, from a more general
curvature preservation condition, we derived that nonnegative Ricci
curvature is preserved for any complete solution of 3-dim Ricci flow. A local
version of Hamilton-Ivey estimates is also obtained. Using that the nonnegative
Ricci is preserved under any 3-dim Ricci flow complete solution, we can prove the strong uniqueness of the Ricci flow with bounded nonnegative Ricci curvature and uniform injective radius lower bound as initial assumptions. This is joint work with Bing-Long Chen and Zhuhong Zhang.

We consider non-infinitesimal deformations of G2-structures on 7-dimensional
manifolds and derive a closed expression for the torsion of the deformed
G2-structure. We then specialize to the case where the deformation lies in
the seven-dimensional representation of G2 and is hence defined by a vector
v. In this case, we explicitly derive the expressions for the different
torsion components of the new G2-structure in terms of the old torsion
components and derivatives of v. In particular this gives a set of
differential equations for the vector v which have to be satisfied for a
transition between G2-structures with particular torsions. For some specific
torsion classes we then explore the solutions of these equations.

Fundamental groups of 3-manifolds are known to satisfy strong
properties, and in recent years there have been several advances in their
study. In this talk I will discuss how some of these properties can be
exploited to give us insight (and results) in the study of 4-manifolds.