# Complexity of parabolic systems

## TL;DR

This paper establishes bounds on the complexity of ancient mean curvature flows using entropy, leading to new insights into the structure and classification of singularities and shrinkers in higher codimension.

## Contribution

It provides the first general bounds on generic singularities in arbitrary codimension and extends classification results for ancient flows that are cylindrical at minus infinity.

## Key findings

- Bound the codimension of ancient flows by entropy.
- All blowups lie in a Euclidean subspace with bounded dimension.
- Rigidity of cylinders as shrinkers in all dimensions and codimensions.

## Abstract

We first bound the codimension of an ancient mean curvature flow by the entropy. As a consequence, all blowups lie in a Euclidean subspace whose dimension is bounded by the entropy and dimension of the evolving submanifolds. This drastically reduces the complexity of the system. Combined with \cite{CM12}, this gives the first general bounds on generic singularities of surfaces in arbitrary codimension. We also show sharp bounds for codimension in arguably some of the most important situations of ancient flows. Namely, we prove that in any dimension and codimension any ancient flow that is cylindrical at $-\infty$ must be a flow of hypersurfaces in a Euclidean subspace. This extends well-known classification results to higher codimension. The bound on the codimension in terms of the entropy is a special case of sharp bounds for spectral counting functions for shrinkers and, more generally, ancient flows. Shrinkers are solutions that evolve by scaling and are the singularity models for the flow. Finally, we show rigidity of cylinders as shrinkers in all dimension and all codimension in a very strong sense: Any shrinker, even in a large dimensional space, that is sufficiently close to a cylinder on a large enough, but compact, set is itself a cylinder. This is an important tool in the theory and is key for regularity; cf. \cite{CM8}.