In this text we outline the major techniques, concepts and results in mean curvature flow with a focus on higher codimension. In addition we include a few novel results and some material that cannot be found elsewhere.
Mean curvature flowMean curvature flow is perhaps the most important geometric evolution equation of submanifolds in Riemannian manifolds. Intuitively, a family of smooth submanifolds evolves under mean curvature flow, if the velocity at each point of the submanifold is given by the mean curvature vector at that point. For example, round spheres in euclidean space evolve under mean curvature flow while concentrically shrinking inward until they collapse in finite time to a single point, the common center of the spheres. Mullins [Mul56] proposed mean curvature flow to model the formation of grain boundaries in annealing metals. Later the evolution of submanifolds by their mean curvature has been studied by Brakke [Bra78] from the viewpoint of geometric measure theory. Among the first authors who studied the corresponding nonparametric problem were Temam [Tem76] in the late 1970's and Gerhardt [Ger80] and Ecker [Eck82] in the early 1980's. Pioneering work was done by Gage [Gag84], Gage & Hamilton [GH86] and Grayson [Gra87] who proved that the curve shortening flow (more precisely, the "mean" curvature flow of curves in R 2 ) shrinks embedded closed curves to "round" points. In his seminal paper Huisken [Hui84] proved that closed convex hypersurfaces in euclidean space R m+1 , m > 1 contract to single round points in finite time (later he extended his result to hypersurfaces in Riemannian manifolds that satisfy a suitable stronger convexity, see [Hui86]). Then, until the mid 1990's, most authors who studied mean curvature flow mainly considered hypersurfaces, both in euclidean and Riemannian manifolds, whereas mean curvature flow in higher codimension did not play a great role. There are various reasons for this, one of them is certainly the much different geometric situation of submanifolds in higher codimension since the normal bundle and the second fundamental tensor