Convex solutions to the mean curvature flow

In this paper we study the classification of ancient convex solutions to the mean curvature flow in R n+1 . An open problem related to the classification of type II singularities is whether a convex translating solution is k-rotationally symmetric for some integer 2 k n, namely whether its level set is a sphere or cylinder S k-1 R n-k . In this paper we give an affirmative answer for entire solutions in dimension 2. In high dimensions we prove that there exist nonrotationally symmetric, entire convex translating solutions, but the blow-down in space of any entire convex translating solution is k-rotationally symmetric. We also prove that the blow-down in space-time of an ancient convex solution which sweeps the whole space R n+1 is a shrinking sphere or cylinder.