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Albert Chau

Publications and source records attributed to Albert Chau.

At least 19 recordsLinked to original sources

Bartnik Mass of CMC surfaces under a Spectral non-negativity condition

Let $g$ be a smooth Riemannian metric and $H$ a positive function on $\mathbb{S}^2$. We prove that the Bartnik mass of the triple $(\mathbb{S}^2,g,H)$ is bounded above by $\sqrt{|\mathbb{S}^2|_g /16\pi}$ provided the first eigenvalue $\lambda_1(g)$ of the operator $(-\Delta_g+K_g)$ is non-negative. We prove a similar result assuming $H$ is only nonnegative and $\lambda_1(g)>0$. These eigenvalue conditions, in particular, impose no lower bound on $K_g$ (even under an area constraint \cite{Mantoulidis_2015} \S3) and thereby extend previous results which assume $K_g\geq 0$.

math.DG

A note on Ricci flow from small curvature concentration and a Morrey-type condition

In \cite{ChauMartens} the authors proved the long-time existence of Ricci flow starting from complete bounded curvature Riemannian manifolds with scale-invariant integral curvature bounded by a dimensional constant times the inverse of the Sobolev constant. We generalize this result by replacing the bounded curvature assumption with the assumption that $g$ is only equivalent to a complete bounded curvature metric $h$ while satisfying a Morrey-type condition on the gradient of $g$ relative to $h$: a local integral condition on the covariant derivative $\nabla_h g$. The Morrey-type condition was first considered in \cite{LeeLiu} in the context of Ricci flow on non-compact manifolds, and in particular allows the possibility for $g$ to have unbounded curvature on $M$. As in \cite{ChauMartens}, our long-time solution enjoys curvature decay estimates implying in particular that $M$ is diffeomorphic to $\mathbb{R}^n$.

math.DG

Long-time Ricci flow existence and topological rigidity from manifolds with pinched scale-invariant integral curvature

We prove long-time existence of the Ricci flow starting from complete manifolds with bounded curvature and scale-invariant integral curvature sufficiently pinched with respect to the inverse of its Sobolev constant. Moreover, if the curvature is sub-critical $L^p$-integrable, this flow converges locally smoothly to a limiting metric $g(\infty)$ on $M$ with $(M,g(\infty))$ isometric to the standard flat $\mathbb{R}^n$, which implies topological rigidity of $M$. This generalizes work of Chen \cite{ChenEric}, who proved analogous results for asymptotically flat manifolds. We also prove a long-time Ricci flow existence (and likewise topological rigidity) result for unbounded curvature initial data, assuming the initial data is a locally smooth limit of bounded curvature manifolds as described above.

math.DG

Pseudolocality and completeness for nonnegative Ricci curvature limits of 3D singular Ricci flows

Lai (2021) used singular Ricci flows, introduced by Kleiner and Lott (2017), to construct a nonnegative Ricci curvature Ricci flow $g(t)$ emerging from an arbitrary 3D complete noncompact Riemannian manifold $(M^3, g_0)$ which has nonnegative Ricci curvature. We show $g(t)$ is complete for positive times provided $g_0$ satisfies a volume ratio lower bound that approaches zero at spatial infinity. Our proof combines a pseudolocality result of Lai (2021) for singular flows, together with a pseudolocality result of Hochard (2016) and Simon and Topping (2022) for nonsingular flows. We also show that the construction of complete nonnegative complex sectional curvature flows by Cabezas-Rivas and Wilking (2015) can be adapted here to show $g(t)$ is complete for positive times provided $g_0$ is a compactly supported perturbation of a nonnegative sectional curvature metric on $\mathbb{R}^3$.

math.DG

Concavity of solutions to semilinear equations in dimension two

We consider the Dirichlet problem for a class of semilinear equations on two dimensional convex domains. We give a sufficient condition for the solution to be concave. Our condition uses comparison with ellipses, and is motivated by an idea of Kosmodem'yanskii. We also prove a result on propagation of concavity of solutions from the boundary, which holds in all dimensions.

math.AP

Strong space-time convexity and the heat equation

We prove local strong convexity of the space-time level sets of the heat equation on convex rings for zero initial data, strengthening a result of Borell. Our proof introduces a parabolic version of a two-point maximum principle of Rosay-Rudin.

math.AP

On the Bartnik mass of non-negatively curved CMC spheres

Let $g$ be a smooth Riemannian metric on $\mathbb{S}^2$ and $H>0$ a constant. We establish an upper bound for the corresponding Bartnik mass $\mathfrak m_B(\mathbb{S}^2, g, H)$ assuming that the Gauss curvature $K_g$ is non-negative. Our upper bound approaches the Hawking mass $\mathfrak m_H(\mathbb{S}^2, g, H)$ when either $g$ becomes round or else $H\to 0$, the bound is zero for $H$ sufficiently large, and in any case the bound is not more than $r/2=\mathfrak m_H(\mathbb{S}^2, g, 0)$. We obtain upper bounds on $\mathfrak m_B(\mathbb{S}^2, g, H)$ as well in the case when $g$ is arbitrary and $H$ is sufficiently large depending on $g$.

math.DG

Exterior Schwarzschild initial data for degenerate apparent horizons

In this note we show that if $g$ is a smooth Riemannian metric on $\mathbb{S}^2$ such that the first eigenvalue of the operator $L_g:=-Δ_g +K_g$ satisfies $λ_1(L_g)=0$ then $(\mathbb{S}^2, g)$ arises as an apparent horizon in an asymptotically flat initial data set with ADM mass arbitrarily close to the associated Hawking mass $\sqrt{\text{area}(\mathbb{S}^2, g)/16π}$. In particular, this determines the Bartnik quasilocal mass (introduced by Bartnik \cite{Bartnik} in 1989) associated with $(\mathbb{S}^2, g)$ in this setting. We prove these by modifying the construction of Mantoulidis-Schoen \cite{MS} who proved the same results in the case $λ_1(L_g)>0$. It follows that $λ_1(g)\geq 0$ is necessary and sufficient for $(\mathbb{S}^2, g)$ to arise from an apparent horizon in an asyptotically flat space-time under the dominant energy condition and in the time symmetric setting, and that the Bartnik mass of the horizon is $\sqrt{\text{area}(\mathbb{S}^2, g)/16π}$.

math.DG

Non-preservation of $\alpha$-concavity for the porous medium equation

We show that the porous medium equation does not in general preserve $\alpha$-concavity of the pressure for $0\le\alpha<1/2$ or $1/2<\alpha\le 1$. In particular, this resolves an open problem of V\'azquez on whether concavity of pressure is preserved by the porous medium equation. Our results strengthen an earlier work of Ishige-Salani, who considered the case of small $\alpha>0$. Since Daskalopoulos-Hamilton-Lee showed that $1/2$-concavity is preserved, our result is sharp. Our explicit examples show that concavity can be instantaneously broken at an interior point of the support of the initial data. For $0\le\alpha<1/2$, we give another set of examples to show that concavity can be broken at a boundary point.

math.AP

The Stefan problem and concavity

We construct examples for the one-phase Stefan problem which show that $α$-concavity of the solution is in general not preserved in time, for $0 \le α<1/2$. In particular, this shows that, in contrast to the case of the heat equation for a fixed convex domain, log concavity is not preserved for solutions of the Stefan problem.

math.AP

The K"ahler-Ricci flow with Log Canonical Singularities

We establish the existence of the K"ahler-Ricci flow on projective varieties with log canonical singularities. This generalizes some of the existence results of Song-Tian \cite{ST3} in case of projective varieties with klt singularities. We also prove that the normalized K"ahler-Ricci flow will converge to the \ka-Einstein metric with negative Ricci curvature on semi-log canonical models in the sense of currents. Finally we also construct K"ahler-Ricci flow solutions performing divisorial contractions and flips with log canonical singularities.

math.DG

The Kähler Ricci flow around complete bounded curvature Kähler metrics

We produce complete bounded curvature solutions to Kähler-Ricci flow with existence time estimates, assuming only that the initial data is a smooth \K metric uniformly equivalent to another complete bounded curvature \K metric. We obtain related flow results for non-smooth as well as degenerate initial conditions. We also obtain a stability result for complex space forms under the flow.

math.DG

Kähler-Ricci flow of cusp singularities on quasi projective varieties

Let $\overline{M}$ be a compact complex manifold with smooth Kähler metric $η$, and let $D$ be a smooth divisor on $\overline{M}$. Let $M=\overline{M}\setminus D$ and let $\hatω$ be a Carlson-Griffiths type metric on $M$. We study complete solutions to Kähler-Ricci flow on $M$ which are comparable to $\hatω$, starting from a smooth initial metric $ω_0=η+i\partial \bar{\partial} ϕ_0$ where $ϕ_0\in C^{\infty}(M)$. When $ω_0\geq c \hatω$ on $M$ for some $c>0$ and $ϕ_0$ has zero Lelong number, we construct a smooth solution $ω(t)$ to Kähler-Ricci flow on $M\times [0, T_{[ω_0 ]})$ where $T_{[ω_0 ]}:= \sup \{ T: [η] +T (c_1(K_{\overline{M}}) + c_1(\mathcal{O}_D))\in \mathcal{K}_M \}$ so that $ω(t)\geq (\frac{1}{n} - \frac{4\hat{K}t}{c} )\hatω$ for all $t\leq \frac{c}{4n\hat{K}}$ where $\hat{K}$ is a non-negative upper bound on the bisectional curvatures of $\hatω$ (see Theorem 1.2). In particular, we do not assume $ω_0$ has bounded curvature. If $ω_0$ has bounded curvature and is asymptotic to $\hatω$ in an appropriate sense, we construct a complete bounded curvature solution on $M\times [0, T_{[ω_0 ]})$ (see Theorem 1.3). These generalize some of the results of Lott-Zhang in [15]. On the other hand if we only assume $ω_0\geq c η$ on $M$ for some $c>0$ and $ϕ_0$ is bounded on $M$, we construct a smooth solution to Kähler-Ricci on $M\times [0, T_{[ω_0 ]})$ which is equivalent to $\hatω$ for all positive times. This includes as a special case when $ω_0$ is smooth on $\overline{M}$ in which case the solution becomes instantaneously complete on $M$ under Kähler-Ricci flow (see Theorem 1.1).

math.DG

An existence time estimate for Kähler-Ricci flow

Fix a complete noncompact \K manifold $(M^n,h_0)$ with bounded curvature. Let $g(t)$ be a bounded curvature solution to the \KR flow starting from some $g_0$ uniformly equivalent to $h_0$. We estimate the existence time of $g(t)$ together with $C^0$ bounds and curvature bounds, where the estimates depend only on $h_0$ and the $C^0$ distance between $g_0$ and $h_0$. We also generalize these results to cases when $g_0$ may have unbounded curvature.

math.DG