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Xian-Tao Huang

Publications and source records attributed to Xian-Tao Huang.

12 recordsLinked to original sources
arXiv2026

Fibrations, the First Betti Number, and Almost Nonnegative Ricci Curvature

In this paper, we prove fibration theorems for manifolds with almost nonnegative Ricci curvature and certain extra regularity assumptions. We show that a closed $n$-manifold $M$ satisfying $\mathrm{diam}(M)^2\mathrm{sec}_M \geq -κ$ and $\mathrm{diam}(M)^2\mathrm{Ric}_M \geq -δ$, where $δ>0$ is sufficiently small depending only on $n$ and $κ$, fibers over a $b_1(M)$-torus. This removes the upper sectional curvature bound required in the earlier result of Yamaguchi \cite{Y88}. As a corollary, we obtain a refinement of Yamaguchi's smooth fibration theorem (\cite{Y91}), showing that the fiber itself (rather than a finite cover of it) fibers over a $b_1$-torus. Our results extend to manifolds satisfying a generalized Reifenberg condition introduced in \cite{HH24}, which encompasses both a lower bound on sectional curvature and the local rewinding Reifenberg condition. In the nonsmooth setting, a similar result also holds for a non-collapsed $\mathrm{RCD}(-ε(D,r,n),n)$ space whose diameter is bounded by $D$ and which satisfies the $(r,δ(n))$-local rewinding Reifenberg condition. The proofs rely on an equivariant regularity theorem for almost submetries under a lower Ricci curvature bound. In addition, we study the stability of rank of Abelian actions along equivariant Gromov-Hausdorff convergence in this paper.

math.DG↗
arXiv2025

Splitting and Slow Volume Growth for Open Manifolds with Nonnegative Ricci Curvature

In \cite{NPZ24}, Navarro-Pan-Zhu proved that the fundamental group of an open manifold with nonnegative Ricci curvature and linear volume growth contains a subgroup isomorphic to $\mathbb{Z}^k$ with finite index. They further asked whether the existence of a torsion-free element in the fundamental group forces the universal cover to split off an isometric $\mathbb{R}$-factor (Question 1.3 of \cite{NPZ24}). In this article, we provide an affirmative answer to this question. Specifically, we prove that if an open manifold with nonnegative Ricci curvature has linear volume growth, then its universal cover is isometric to a metric product $\mathbb{R}^k \times N$, where $N$ is an open manifold with linear volume growth and $k$ is the integer such that $π_1(M)$ contains a $\mathbb{Z}^k$-subgroup of finite index. As a direct consequence, if the Ricci curvature is positive at some point, then the fundamental group is finite. We also establish that for an open manifold $M$ with nonnegative Ricci curvature, if the infimum of its volume growth order is strictly less than $3$ and $\tilde{M}$ has Euclidean volume growth, then the universal cover $\tilde{M}$ splits off an $\mathbb{R}^{n-2}$-factor. As an application, if $M$ has first Betti number $b_1 = n-2$ and $\tilde{M}$ has Euclidean volume growth, then its universal cover admits such a splitting. This result provides a partial answer to \cite[Question 1.6]{PY24}.

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arXiv2025

Nonnegative Ricci Curvature, Euclidean Volume Growth, and the Fundamental Groups of Open $4$-Manifolds

Let $M$ be a 4-dimensional open manifold with nonnegative Ricci curvature. In this paper, we prove that if the universal cover of $M$ has Euclidean volume growth, then the fundamental group $π_1(M)$ is finitely generated. This result confirms Pan-Rong's conjecture \cite{PR18} for dimension $n = 4$. Additionally, we prove that there exists a universal constant $C>0$ such that $π_1(M)$ contains an abelian subgroup of index $\le C$. More specifically, if $π_1(M)$ is infinite, then $π_1(M)$ is a crystallographic group of rank $\le 3$. If $π_1(M)$ is finite, then $π_1(M)$ is isomorphic to a quotient of the fundamental group of a spherical 3-manifold.

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arXiv2024

Almost splitting maps, transformation theorems and smooth fibration theorems

In this paper, we introduce a notion, called generalized Reifenberg condition, under which we prove a smooth fibration theorem for collapsed manifolds with Ricci curvature bounded below, which gives a unified proof of smooth fibration theorems in many previous works (including the ones proved by Fukaya and Yamaguchi respectively). A key tool in the proof of this fibration theorem is the transformation technique for almost splitting maps, which originates from Cheeger-Naber (\cite{CN}) and Cheeger-Jiang-Naber (\cite{CJN21}). More precisely, we show that a transformation theorem of Cheeger-Jiang-Naber (see Proposition 7.7 in \cite{CJN21}) holds for possibly collapsed manifolds. Some other applications of the transformation theorems are given in this paper.

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arXiv2024

Optimal asymptotic volume ratio for noncompact 3-manifolds with asymptotically nonnegative Ricci curvature and a uniformly positive scalar curvature lower bound

In this paper, we study 3-dimensional complete non-compact Riemannian manifolds with asymptotically nonnegative Ricci curvature and a uniformly positive scalar curvature lower bound. Our main result is that, if this manifold has $k$ ends and finite first Betti number, then it has at most linear volume growth, and furthermore, if the negative part of Ricci curvature decays sufficiently fast at infinity, then we have an optimal asymptotic volume ratio $\limsup_{r\rightarrow\infty}\frac{\mathrm{Vol}(B(p, r))}{r}\leq4kπ$. In particular, our results apply to 3-dimensional complete non-compact Riemannian manifolds with nonnegative Ricci curvature and a uniformly positive scalar curvature lower bound.

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arXiv2021

Harmonic functions with polynomial growth on manifolds with nonnegative Ricci curvature

Suppose $(M,g)$ is a Riemannian manifold having dimension $n$, nonnegative Ricci curvature, maximal volume growth and unique tangent cone at infinity. In this case, the tangent cone at infinity $C(X)$ is an Euclidean cone over the cross-section $X$. Denote by $α=\lim_{r\rightarrow\infty}\frac{\mathrm{Vol}(B_{r}(p))}{r^{n}}$ the asymptotic volume ratio. Let $h_{k}=h_{k}(M)$ be the dimension of the space of harmonic functions with polynomial growth of growth order at most $k$. In this paper, we prove a upper bound of $h_{k}$ in terms of the counting function of eigenvalues of $X$. As a corollary, we obtain $\lim_{k\rightarrow\infty}k^{1-n}h_{k}=\frac{2α}{(n-1)!ω_{n}}$. These results are sharp, as they recover the corresponding well-known properties of $h_{k}(\mathbb{R}^{n})$. In particular, these results hold on manifolds with nonnegative sectional curvature and maximal volume growth.

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arXiv2017

An almost rigidity Theorem and its applications to noncompact RCD(0,N) spaces with linear volume growth

The main results of this paper consists of two parts. Firstly, we obtain an almost rigidity theorem which says that on a RCD(0, N) space, when a domain between two level sets of a distance function has almost maximal volume compared to that of a cylinder, then this portion is close to a cylinder as a metric space. Secondly, we apply this almost rigidity theorem to study noncompact RCD(0, N) spaces with linear volume growth. More precisely, we obtain the sublinear growth of diameter of geodesic spheres, and study the non-existence of harmonic functions with polynomial growth on such RCD(0,N) spaces.

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arXiv2017

On the asymptotic behavior of the dimension of spaces of harmonic functions with polynomial growth

Suppose $(M^{n},g)$ is a Riemannian manifold with nonnegative Ricci curvature, and let $h_{d}(M)$ be the dimension of the space of harmonic functions with polynomial growth of growth order at most $d$. Colding and Minicozzi proved that $h_{d}(M)$ is finite. Later on, there are many researches which give better estimates of $h_{d}(M)$. We study the behavior of $h_{d}(M)$ when $d$ is large in this paper. More precisely, suppose that $(M^{n},g)$ has maximal volume growth and has a unique tangent cone at infinity, then when $d$ is sufficiently large, we obtain some estimates of $h_{d}(M)$ in terms of the growth order $d$, the dimension $n$ and the the asymptotic volume ratio $α=\lim_{R\rightarrow\infty}\frac{\mathrm{Vol}(B_{p}(R))}{R^{n}}$. When $α=ω_{n}$, i.e., $(M^{n},g)$ is isometric to the Euclidean space, the asymptotic behavior obtained in this paper recovers a well-known asymptotic property of $h_{d}(\mathbb{R}^{n})$.

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arXiv2014

A Simple Method for the Optimal Transportation

In this paper we will give a new proof of the monotonicity of Wasserstein distances of two diffusions under super Ricci flow. Our proof is based on the coupling method of B.Andrew and J.Clutterbuck. The same method can also be applied to the contractivity of normalized L-Wasserstein distance under backward Ricci flow.

math.DG↗
arXiv2014

A note on the positivity of a quasi-local mass in general dimensions

Wang and Yau [10] introduced a quasi-local mass, which is a hyperbolic background generalization of Liu-Yau's expression [7] [8], and proved its positivity. In this note, we prove that the positivity of this quasi-local mass is still valid under weaker assumptions on the boundary hypersurface in general dimensions. The method we used is similar to that used by Eichmair, Miao and Wang in [4].

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