The topology of hypersurfaces moving by mean curvature.

*(English)*Zbl 0858.58047The levels \(M_t\) of the mean curvature flow may become singular as \(t\to T\). Beyond the parameter \(T\) there are still weak solutions, the so-called weak mean curvature flow. This paper addresses the question: what topological changes of \(M_t\) are possible? Roughly, the main result states that two distinct connected components of the complement \(W [0]\) of \(M_0\) cannot become connected together later, and that connected components cannot appear out of nowhere. This is expressed by an induced isomorphism in the homology \(H_0 (W [0]) \to H_0 (W[0,T])\). Similarly, every loop in \(W[0,T]\) is homotopic to a loop in \(W[0]\). Furthermore, if the ambient space is an \((n+1)\)-dimensional Riemannian manifold with Ricci curvature bounded below, then there is a similar result for the homologies \(H_n\) and \(H_{n-1}\). A number of examples illustrate various topology changes which actually can occur after singularities of the flow.

Reviewer: W.Kühnel (Stuttgart)