# Tobias Holck ColdingMassachusetts Institute of Technology | MIT · Department of Mathematics

Tobias Holck Colding

PhD

## About

126

Publications

16,169

Reads

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7,010

Citations

Citations since 2017

Additional affiliations

January 2016 - July 2016

**Mathematical Sciences Research Institute**

Position

- Professor

August 2011 - January 2012

**Mathematical Sciences Research Institute**

Position

- Eisenbud Professor

January 2005 - present

## Publications

Publications (126)

Dedicated to Blaine Lawson with admiration. Abstract. Comparing and recognizing metrics can be extraordinarily difficult because of the group of diffeomorphisms. Two metrics, that could even be the same, could look completely different in different coordinates. This is the gauge problem. The general gauge problem is extremely subtle, especially for...

Comparing and recognizing metrics can be extraordinarily difficult because of the group of diffeomorphisms. Two metrics, that could even be the same, could look completely different in different coordinates. For compact manifolds, various techniques exist for understanding this. However, for non-compact manifolds, no general techniques exist; contr...

Analysis of non-compact manifolds almost always requires some controlled behavior at infinity. Without such, one neither can show, nor expect, strong properties. On the other hand, such assumptions restrict the possible applications and often too severely. In a wide range of areas non-compact spaces come with a Gaussian weight and a drift Laplacian...

We prove monotonicity of a parabolic frequency on static and evolving manifolds without any curvature or other assumptions. These are parabolic analogs of Almgren’s frequency function. When the static manifold is Euclidean space and the drift operator is the Ornstein–Uhlenbeck operator, this can been seen to imply Poon’s frequency monotonicity for...

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. We use this in a major application of our new methods to give the firs...

By a classical result, solutions of analytic elliptic PDEs, like the Laplace equation, are analytic. In many instances, the properties that come from being analytic are more important than analyticity itself. Many important equations are degenerate elliptic and solutions have much lower regularity. Still, one may hope that solutions share propertie...

We prove monotonicity of a parabolic frequency on manifolds. This is a parabolic analog of Almgren's frequency function. Remarkably we get monotonicity on all manifolds and no curvature assumption is needed. When the manifold is Euclidean space and the drift operator is the Ornstein-Uhlenbeck operator this can been seen to imply Poon's frequency mo...

We survey some recent geometric methods for studying Heegaard splittings of 3-manifolds

We will look for stable structures in four situations and discuss what is known and unknown.

We show that all closed $2$-dimensional singularities for higher codimension mean curvature flow that cannot be perturbed away have uniform entropy bounds and lie in a linear subspace of small dimension. The entropy and dimension of the subspace are both $\leq C\,(1+\gamma)$ for some universal constant $C$ and genus $\gamma$.

We show uniqueness of cylindrical blowups for mean curvature flow in all dimension and all codimension. Mean curvature flow in higher codimension is a nonlinear parabolic system whose complexity increases as the codimension increases. Our results imply regularity of the singular set for the system.

We bound the codimension of an ancient mean curvature flow by the entropy. As a consequence, all blowups near a singularity 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. The bound on the codimension is a special case of sharp b...

The classical Liouville theorem states that a bounded harmonic function on all of $\RR^n$ must be constant. In the early 1970s, S.T. Yau vastly generalized this, showing that it holds for manifolds with nonnegative Ricci curvature. Moreover, he conjectured a stronger Liouville property that has generated many significant developments. We will first...

For any manifold with polynomial volume growth, we show: The dimension of the space of ancient caloric functions with polynomial growth is bounded by the degree of growth times the dimension of harmonic functions with the same growth. As a consequence, we get a sharp bound for the dimension of ancient caloric functions on any space where Yau's 1974...

The classical Liouville theorem states that a bounded harmonic function on allofRnmust be constant. In the early 1970s, S.T. Yau vastly generalized this, showing that itholds for manifolds with nonnegative Ricci curvature. Moreover, he conjectured a strongerLiouville property that has generated many significant developments. We will first discussth...

Parabolic geometric flows are smoothing for short time however, over long time, singularities are typically unavoidable, can be very nasty and may be impossible to classify. The idea of [CM6] and here is that, by bringing in the dynamical properties of the flow, we obtain also smoothing for large time for generic initial conditions. When combined w...

Parabolic geometric flows are smoothing for short time however, over long time, singularities are typically unavoidable, can be very nasty and may be impossible to classify. The idea of [CM6] and here is that, by bringing in the dynamical properties of the flow, we obtain also smoothing for large time for generic initial conditions. When combined w...

We prove sharp bounds for the growth rate of eigenfunctions of the Ornstein-Uhlenbeck operator and its natural generalizations. The bounds are sharp even up to lower order terms and have important applications to geometric flows.

Parabolic geometric flows have the property of smoothing for short time however, over long time, singularities are typically unavoidable, can be very nasty and may be impossible to classify. The idea of this paper is that, by bringing in the dynamical properties of the flow, we obtain also smoothing for long time for generic initial conditions. Whe...

By a classical result, solutions of analytic elliptic PDEs, like the Laplace equation, are analytic. In many instances, the properties that come from being analytic are more important than analyticity itself. Many important equations are degenerate elliptic and solutions have much lower regularity. Still, one may hope that solutions share propertie...

We prove conjectures of René Thom and Vladimir Arnold for C 2 solutions to the degenerate elliptic equation that is the level set equation for motion by mean curvature.
We believe these results are the first instances of a general principle: Solutions of many degenerate equations behave as if they are analytic, even when they are not. If so, this...

We prove conjectures of René Thom and Vladimir Arnold for C² solutions to the degenerate elliptic equation that is the level set equation for motion by mean curvature.
We believe these results are the first instances of a general principle: Solutions of many degenerate equations behave as if they are analytic, even when they are not. If so, this wo...

Modeling of a wide class of physical phenomena, such as crystal growth and flame propagation, leads to tracking fronts moving with curvature-dependent speed. When the speed is the curvature this leads to one of the classical degenerate nonlinear second order differential equations on Euclidean space. One naturally wonders "what is the regularity of...

We will be concerned with the regularity of solutions to a classical degenerate nonlinear second order differential equation on Euclidean space. A priori solutions were only defined in a weak sense but it turns out that they are always twice differentiable classical solutions. The proof weaves together analysis and geometry. Without deeply understa...

We showed earlier that the level set function of a monotonic advancing front is twice differentiable everywhere with bounded second derivative. We show here that the second derivative is continuous if and only if the flow has a single singular time where it becomes extinct and the singular set consists of a closed $C^1$ manifold with cylindrical si...

The long standing classification problem in the theory of Heegaard splittings
of 3-manifolds is to exhibit for each closed 3-manifold a complete list,
without duplication, of all its irreducible Heegaard surfaces, up to isotopy.
We solve this problem for non Haken hyperbolic 3-manifolds.

This paper deals with interactions between committee members as they rank a
large list of applicants for a given position and eventually reach consensus.
We will see that for a natural deterministic model the ranking can be described
by solutions of a discrete quasilinear heat equation with time dependent
coefficients on a graph.
We show first that...

Once one knows that singularities occur, one naturally wonders what the singularities are like. For minimal varieties the first answer, already known to Federer-Fleming in 1959, is that they weakly resemble cones. For mean curvature flow, by the combined work of Huisken, Ilmanen, and White, singularities weakly resemble shrinkers. Unfortunately, th...

Mean curvature flow is the negative gradient flow of volume, so any hypersurface flows through hypersurfaces in the direction of steepest descent for volume and eventually becomes extinct in finite time. Before it becomes extinct, topological changes can occur as it goes through singularities. If the hypersurface is in general or generic position,...

For a monotonically advancing front, the arrival time is the time when the
front reaches a given point. We show that it is twice differentiable everywhere
with uniformly bounded second derivative. It is smooth away from the critical
points where the equation is degenerate. We also show that the critical set has
finite codimensional two Hausdorff me...

The main result is a short effective proof of Tao Li's theorem that a closed non-Haken hyperbolic 3-manifolds. N has at most finitely many irreducible Heegaard splittings. Along the way we show that N has finitely many branched surfaces of pinched negative sectional curvature carrying all closed index-≤ 1 minimal surfaces. This effective result, to...

We show that for a mean curvature flow of closed embedded hypersurfaces in
$\bf{R}^{n+1}$ with only generic singularities the space-time singular set is
contained in finitely many compact embedded $(n-1)$-dimensional Lipschitz
submanifolds plus a set of dimension at most $n-2$. If the initial hypersurface
is mean convex, then all singularities are...

In real algebraic geometry, Lojasiewicz's theorem asserts that any integral
curve of the gradient flow of an analytic function that has an accumulation
point has a unique limit. Lojasiewicz proved this result in the early 1960s as
a consequence of his gradient inequality.
Many problems in calculus of variations are questions about critical points
o...

Once one knows that singularities occur, one naturally wonders what the
singularities are like. For minimal varieties the first answer, already known
to Federer-Fleming in 1959, is that they weakly resemble cones. For mean
curvature flow, by the combined work of Huisken, Ilmanen, and White,
singularities weakly resemble shrinkers. Unfortunately, th...

Shrinkers are special solutions of mean curvature flow (MCF) that evolve by
rescaling and model the singularities. While there are infinitely many in each
dimension, [CM1] showed that the only generic are round cylinders $\SS^k\times
\RR^{n-k}$. We prove here that round cylinders are rigid in a very strong
sense. Namely, any other shrinker that is...

In this paper we generalize the monotonicity formulas of “Colding (Acta Math 209:229–263, 2012)” for manifolds with nonnegative Ricci curvature. Monotone quantities play a key role in analysis and geometry; see, e.g., “Almgren (Preprint)”, “Colding and Minicozzi II (PNAS, 2012)”, “Garofalo and Lin (Indiana Univ Math 35:245–267, 1986)” for applicati...

In this paper we will discuss how one may be able to use mean curvature flow
to tackle some of the central problems in topology in 4-dimensions.
We will be concerned with smooth closed 4-manifolds that can be smoothly
embedded as a hypersurface in R^5. We begin with explaining why all closed
smooth homotopy spheres can be smoothly embedded. After t...

In this expository article, we discuss various monotonicity formulas for parabolic and elliptic operators and explain how the analysis of function spaces and the geometry of the underlining spaces are intertwined. After briefly discussing some of the well-known analytical applications of monotonicity for parabolic operators, we turn to their ellipt...

We show that for any Ricci-flat manifold with Euclidean volume growth the
tangent cone at infinity is unique if one tangent cone has a smooth
cross-section. Similarly, for any noncollapsing limit of Einstein manifolds
with uniformly bounded Einstein constants, we show that local tangent cones are
unique if one tangent cone has a smooth cross-sectio...

The entropy of a hypersurface is a geometric invariant that measures
complexity and is invariant under rigid motions and dilations. It is given by
the supremum over all Gaussian integrals with varying centers and scales. It is
monotone under mean curvature flow, thus giving a Lyapunov functional.
Therefore, the entropy of the initial hypersurface b...

It has long been conjectured that starting at a generic smooth closed embedded surface in R^3, the mean curvature flow remains smooth until it arrives at a singularity in a neighborhood of which the flow looks like concentric spheres or cylinders. That is, the only singularities of a generic flow are spherical or cylindrical. We will address this c...

We prove three new monotonicity formulas for manifolds with a lower Ricci curvature bound and show that they are connected to rate of convergence to tangent cones. In fact, we show that the derivative of each of these three monotone quantities is bounded from below in terms of the Gromov–Hausdorff distance to the nearest cone. The monotonicity form...

We study here limit spaces
$(M_\alpha,g_\alpha,p_\alpha)\stackrel{GH}{\rightarrow} (Y,d_Y,p)$, where the
$M_\alpha$ have a lower Ricci curvature bound and are volume noncollapsed. Such
limits $Y$ may be quite singular, however it is known that there is a subset of
full measure $\cR(Y)\subseteq Y$, called {\it regular} points, along with
coverings b...

Consider a limit space
$(M_\alpha,g_\alpha,p_\alpha)\stackrel{GH}{\rightarrow} (Y,d_Y,p)$, where the
$M_\alpha^n$ have a lower Ricci curvature bound and are volume noncollapsed.
The tangent cones of $Y$ at a point $p\in Y$ are known to be metric cones
$C(X)$, however they need not be unique. Let
$\bar\Omega_{Y,p}\subseteq\cM_{GH}$ be the closed sub...

We prove a new estimate on manifolds with a lower Ricci bound which asserts that the geometry of balls centered on a minimizing geodesic can change in at most a Holder continuous way along the geodesic. We give examples that show that the Holder exponent, along with essentially all the other consequences that follow from this estimate, are sharp.
A...

We discuss recent results on minimal surfaces and mean curvature flow,
focusing on the classification and structure of embedded minimal surfaces and
the stable singularities of mean curvature flow. This article is dedicated to
Rick Schoen.

We prove lower bounds for the Hausdorff measure of nodal sets of
eigenfunctions.

We prove a smooth compactness theorem for the space of embedded self-shrinkers in $\RR^3$. Since self-shrinkers model singularities in mean curvature flow, this theorem can be thought of as a compactness result for the space of all singularities and it plays an important role in studying generic mean curvature flow.

We show that for a Schrödinger operator with bounded potential on a manifold with cylindrical ends, the space of solutions that grows at most exponentially at infinity is finite dimensional and, for a dense set of potentials (or, equivalently, for a surface for a fixed potential and a dense set of metrics), the constant function 0 is the only solut...

This is an expository article with complete proofs intended for a general non-specialist audience. The results are two-fold. First, we discuss a geometric invariant, that we call the width, of a manifold and show how it can be realized as the sum of areas of minimal 2-spheres. For instance, when $M$ is a homotopy 3-sphere, the width is loosely spea...

Given a Riemannian metric on a homotopy $n$-sphere, sweep it out by a continuous one-parameter family of closed curves starting and ending at point curves. Pull the sweepout tight by, in a continuous way, pulling each curve as tight as possible yet preserving the sweepout. We show: Each curve in the tightened sweepout whose length is close to the l...

Given a Riemannian metric on the 2-sphere, sweep the 2-sphere out by a continuous one-parameter family of closed curves starting and ending at point curves. Pull the sweepout tight by, in a continuous way, pulling each curve as tight as possible yet preserving the sweepout. We show the following useful property (see Theorem 1.9 below); cf. [CM1], [...

We show that for a Schr\"odinger operator with bounded potential on a manifold with cylindrical ends the space of solutions which grows at most exponentially at infinity is finite dimensional and, for a dense set of potentials (or, equivalently for a surface, for a fixed potential and a dense set of metrics), the constant function zero is the only...

Surfaces that locally minimize area have been extensively used to model physical phenomena, including soap films, black holes, compound polymers, protein folding, etc. The mathematical field dates to the 1740s but has recently become an area of intense mathematical and scientific study, specifically in the areas of molecular engineering, materials...

Minimal surfaces with uniform curvature (or area) bounds have been well understood and the regularity theory is complete, yet essentially nothing was known without such bounds. We discuss here the theory of embedded (i.e., without self-intersections) minimal surfaces in Euclidean 3-space without a priori bounds. The study is divided into three case...

We give a quick tour through many of the classical results in the field of minimal submanifolds, starting at the definition. The field of minimal submanifolds remains extremely active and has very recently seen major developments that have solved many longstanding open problems and conjectures; for more on this, see the expanded version of this sur...

This paper is the fifth and final in a series on embedded minimal surfaces.
Following our earlier papers on disks, we prove here two main structure
theorems for non-simply connected embedded minimal surfaces of any given fixed
genus.
The first of these asserts that any such surface without small necks can be
obtained by gluing together two opposite...

In what follows we give a quick tour through the field of minimal submanifolds, starting at the definition and the classical results and ending up with current areas of research.

For any 3-manifold M and any nonnegative integer g, we give here examples of metrics on M each of which has a sequence of embedded minimal surfaces of genus g and without Morse index bounds. On any spherical space form S^3/Gamma we construct such a metric with positive scalar curvature. More generally we construct such a metric with Scal>0 (and suc...

In this paper we will prove the Calabi-Yau conjectures for embedded surfaces. In fact, we will prove considerably more. The Calabi-Yau conjectures about surfaces date back to the 1960s. Much work has been done on them over the past four decades. In particular, examples of Jorge-Xavier from 1980 and Nadirashvili from 1996 showed that the immersed ve...

It is well known that the size, e.g. Hausdorff measure, of a nodal set of an eigenfunction tends to increase with the eigenvalue. However, because of the highly oscillatory behavior, it is unclear what happens to the volume of the sets where the eigenfunctions are small. In this paper, we show that this volume is bounded uniformly from below for ei...

In this note we announce results on the mean curvature flow of mean convex sets in 3-dimensions. Loosely speaking, our results justify the naive picture of mean curvature flow where the only singularities are neck pinches, and components which collapse to asymptotically round spheres.

In this note we prove some bounds for the extinction time for the Ricci flow on certain 3-manifolds. Our interest in this comes from a question of Grisha Perelman asked to the first author at a dinner in New York City on April 25th of 2003. His question was ``what happens to the Ricci flow on the 3-sphere when one starts with an arbitrary metric? I...

This is a guided tour through some selected topics in geometric analysis. We have chosen to illustrate many of the basic ideas as they apply to the theory of minimal surfaces. This is, in part, because minimal surfaces is, if not the oldest, then certainly one of the oldest areas of geometric analysis dating back to Euler's work in the 1740's and i...

Sharp estimates for mean curvature flow of graphs are shown and examples are given to illustrate why these are sharp. The estimates improves earlier (non-sharp) estimates of Klaus Ecker and Gerhard Huisken.

In this paper we survey with complete proofs some well--known, but hard to find, results about constructing closed embedded minimal surfaces in a closed 3-dimensional manifold via min--max arguments. This includes results of J. Pitts, F. Smith, and L. Simon and F. Smith.

this paper intended for a general nonmathematical audience. The article [A] discusses in a simple nontechnical way the shape of various things that are of "minimal" type. These shapes include soap film and soap bubles, metal alloys, radiolarian skeletons, and embryonic tissues and cells. The reader interested in some of the history of the field of...

We construct a sequence of (compact) embedded minimal disks in a ball in R 3 with boundaries in the boundary of the ball and where the curvatures blow up only at the center. The sequence converges to a limit which is not smooth and not proper. If instead the sequence of embedded disks had boundaries in a sequence of balls with radii tending to infi...

On any surface we give an example of a metric that contains simple closed geodesics with arbitrary high Morse index. Similarly, on any 3-manifold we give an example of a metric that contains embedded minimal tori with arbitrary high Morse index. Previously no such examples were known. We also discuss whether or not such bounds should hold for a gen...

This paper is the third in a series where we describe the space of all embedded minimal surfaces of fixed genus in a fixed (but arbitrary) closed 3-manifold. In [CM3]-[CM5] we describe the case where the surfaces are topologically disks on any fixed small scale. To describe general planar domains (in [CM6]) we need in addition to the results of [CM...

This paper is the fourth in a series where we describe the space of all embedded minimal surfaces of fixed genus in a fixed (but arbitrary) closed 3-manifold. The key is to understand the structure of an embedded minimal disk in a ball in $\RR^3$. This was undertaken in [CM3], [CM4] and the global version of it will be completed here; see [CM15] fo...

This paper is the second in a series where we attempt to give a complete description of the space of all embedded minimal surfaces of fixed genus in a fixed (but arbitrary) closed 3-manifold. The key for understanding such surfaces is to understand the local structure in a ball and in particular the structure of an embedded minimal disk in a ball i...

This paper is the first in a series where we attempt to give a complete description of the space of all embedded minimal surfaces of fixed genus in a fixed (but arbitrary) closed Riemannian 3-manifold. The key for understanding such surfaces is to understand the local structure in a ball and in particular the structure of an embedded minimal disk i...

What are the possible shapes of various things and why?
For instance, when a closed wire or a frame is dipped into a soap solution
and is raised up from the solution, the surface spanning the wire is a soap
film. What are the possible shapes of soap films and why? Or, for instance, why
is DNA like a double spiral staircase? ``What..?'' and ``why..?...

We give in this paper bounds for the Morse indices of a large class of simple geodesics on a surface with a generic metric. To our knowledge these bounds are the first that use only the generic hypothesis on the metric.

This is a survey of our work on embedded minimal disks.

We bound area and total curvature for intrinsic balls in surfaces which are stable. An important feature is that there are
no a priori assumptions on the surface. The fact that the argument applies (and gives good estimates) to more general domains
than intrinsic balls plays a key role in our study (given elsewhere) of general (not necessarily stab...

The point of this paper is two-fold: first, we give new estimates for multi-valued solutions u of the minimal surface equation. Second, we apply these to prove that certain embedded minimal disks are properly embedded.

We show that any embedded minimal annulus in a ball (with boundary in the boundary of the ball and) with a small neck can
be decomposed by a simple closed geodesic into two graphical subannuli. Moreover, we give a sharp bound for the length of
this closed geodesic in terms of the separation (or height) between the graphical subannuli. This will ill...

In this short paper, we apply estimates and ideas from
[CM4] to study the ends of a properly embedded complete
minimal surface
Σ2⊂ℝ3 with finite
topology. The main result is that any complete properly
embedded minimal annulus that lies above a sufficiently narrow
downward sloping cone must have finite total curvature.

In a prior work, we showed that any sequence of uniformly locally simply connected embbeded minimal surfaces in a 3 -manifold has a subsequence which converges outside a closed singular set of finite codimensional two measure to a minimal lamination. We announce here that the singularities are removable in the limit.

We announce various results from an ongoing investigation into convergence of embedded minimal surfaces in three-manifolds.RésumeOn annonce tes premiers résultats d'une recherche en cours sur la convergence des surfaces minimales plongées dans des variétés de dimension 3.