The study of positive sectional curvature is one of the oldest pursuits in Riemannian geometry, but despite the considerable efforts of many researchers, basic questions remain unanswered. In this lecture we will briefly summarize the state of knowledge in this area and outline the techniques which have had success. These techniques include geodesic and comparison methods, minimal surface methods, and Ricci flow. We will then describe our recent work (see [18], [21], [22]) which uses the Ricci flow to resolve the differentiable sphere theorem; that is, the complete classification of manifolds whose sectional curvatures are 1/4-pinched.Mathematics Subject Classification (2010). Primary 53C20; Secondary 53C21, 53C44, 35K55Keywords. Riemann curvature tensor, Ricci flow, strong maximum principle, symmetric space
Preliminaries and the Main TheoremsWe let M denote a smooth manifold of dimension n. Recall that a Riemannian metric on M is a choice g of inner product on each tangent space which varies smoothly from point to point. Any manifold admits an infinite dimensional family of Riemannian metrics, but the question of whether a manifold admits metrics with desired geometric properties is one of the basic questions of global Riemannian geometry. Surfaces embedded in R 3 provide important examples of two dimensional Riemannian manifolds where the metric g is the restriction of the euclidean inner product to each tangent space. The geometry of surfaces was developed by Gauss in the early nineteenth century. Gauss understood