The following was shown by Michael Gage and Richard S. Hamilton as an application of Hamilton's general existence theorem for parabolic geometric flows. Let be a compact smooth manifold, let be a complete smooth Riemannian manifold, and let be a smooth immersion. Then there is a positive number, which could be infinite, and a map with the following properties:
if is any other map with the four properties above, then and for any
Necessarily, the restriction of to is. One refers to as the mean curvature flow with initial data.
Convergence theorems
Following Hamilton's epochal 1982 work on the Ricci flow, in 1984 Gerhard Huisken employed the same methods for the mean curvature flow to produce the following analogous result:
If is the Euclidean space, where denotes the dimension of, then is necessarily finite. If the second fundamental form of the 'initial immersion' is strictly positive, then the second fundamental form of the immersion is also strictly positive for every, and furthermore if one choose the function such that the volume of the Riemannian manifold is independent of, then as the immersions smoothly converge to an immersion whose image in is a round sphere.
Note that if and is a smooth hypersurface immersion whose second fundamental form is positive, then the Gauss map is a diffeomorphism, and so one knows from the start that is diffeomorphic to and, from elementary differential topology, that all immersions considered above are embeddings. Gage and Hamilton extended Huisken's result to the case. Matthew Grayson showed that if is any smooth embedding, then the mean curvature flow with initial data eventually consists exclusively of embeddings with strictly positive curvature, at which point Gage and Hamilton's result applies. In summary:
If is a smooth embedding, then consider the mean curvature flow with initial data. Then is a smooth embedding for every and there exists such that has positive curvature for every. If one selects the function as in Huisken's result, then as the embeddings converge smoothly to an embedding whose image is a round circle.
Physical examples
The most familiar example of mean curvature flow is in the evolution of soap films. A similar 2-dimensional phenomenon is oil drops on the surface of water, which evolve into disks. Mean curvature flow was originally proposed as a model for the formation of grain boundaries in the annealing of pure metal.
Mean curvature flow of a three-dimensional surface
The differential equation for mean-curvature flow of a surface given by is given by with being a constant relating the curvature and the speed of the surface normal, and the mean curvature being In the limits and , so that the surface is nearly planar with its normal nearly parallel to the z axis, this reduces to a diffusion equation While the conventional diffusion equation is a linear parabolic partial differential equation and does not develop singularities, mean curvature flow may develop singularities because it is a nonlinear parabolic equation. In general additional constraints need to be put on a surface to prevent singularities under mean curvature flows. Every smooth convex surface collapses to a point under the mean-curvature flow, without other singularities, and converges to the shape of a sphere as it does so. For surfaces of dimension two or more this is a theorem of Gerhard Huisken; for the one-dimensional curve-shortening flow it is the Gage–Hamilton–Grayson theorem. However, there exist embedded surfaces of two or more dimensions other than the sphere that stay self-similar as they contract to a point under the mean-curvature flow, including the Angenent torus.
