Abstract. The maximum principle is one of the most important tools in the analysis of geometric partial differential equations. Traditionally, the maximum principle is applied to a scalar function defined on a manifold, but in recent years more sophisticated versions have emerged. One particularly interesting direction involves applying the maximum principle to functions that depend on a pair of points. This technique is particularly effective in the study of problems involving embedded surfaces.In this survey, we first describe some foundational results on curve shortening flow and mean curvature flow. We then describe Huisken's work on the curve shortening flow where the method of two-point functions was introduced. Finally, we discuss several recent applications of that technique. These include sharp estimates for mean curvature flow as well as the proof of Lawson's 1970 conjecture concerning minimal tori in S 3 .
Background on minimal surfaces and mean curvature flowMinimal surfaces are among the most important objects studied in differential geometry. A minimal surface is characterized by the fact that it is a critical point for the area functional; in other words, if we deform the surface while keeping the boundary fixed, then the surface area is unchanged to first order. Surfaces with this property serve as mathematical models for soap films in physics.The first variation of the surface area can be expressed in terms of the curvature of the surface. To explain this, let us consider the most basic case of a surface M in R 3 . The curvature of M at a point p ∈ M is described by a symmetric bilinear form defined on the tangent plane T p M , which is referred to as the second fundamental form of M . The eigenvalues of the second fundamental form are the principal curvatures of M . We can think of the principal curvatures as follows: Given any point p ∈ M , we may locally represent the surface M as a graph over the tangent plane T p M . The principal curvatures of M at p can then be interpreted as the eigenvalues of the Hessian of the height function at the point p. For example, for a sphere of radius r, the principal curvatures are both equal to 1 r ; similarly, the principal curvatures of a cylinder of radius r are equal to 1 r and 0. If the principal curvatures of M at the point p have the same sign, then the surface M will be convex or concave near p, depending on the orientation. On the other hand, if the principal curvatures at a point p have opposite signs, then the surface M will look like a saddle locally near p. The sum of the principal curvatures is referred to as the mean curvature of M and is denoted by H.