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Gabjin Yun

Publications and source records attributed to Gabjin Yun.

18 recordsLinked to original sources
arXiv2025

On the structure of almost Yamabe solitons

In this paper, we study structures of almost Yamabe solitons which are not necessarily gradient. First, we investigate conditions that both compact and noncompact almost Yamabe solitons become trivial solitons which means the given vector field is a Killing vector field. Second, we show that an almost Yamabe soliton whose vector field is closed admits a local warped product structure with a one-dimensional base. This result can be considered as a generalization of a result in \cite{c-s-z} and \cite{c-m-m}

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arXiv2023

Vacuum static spaces and Conformal vector fields

In this paper, we show that if a compact $n$-dimensional vacuum static space $(M^n, g, f)$ admits a non-trivial closed conformal vector field $V$, then $(M, g)$ is isometric to a standard sphere ${\Bbb S}^n(c)$. We also prove that if a pair $(g, f)$ of a Riemannian metric and a function defined on a compact $n$-dimensional manifold $M^n$ satisfies the critical point equation and $(M, g)$ admits a non-trivial closed conformal vector field $V$, we have the same result. Finally, we prove a criterion for a nontrivial conformal vector field to be closed.

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arXiv2022

On the geometry of Einstein-type manifolds with some structural conditions

In this paper, we investigate the geometry of Einstein-type equation on a Riemannian manifold, unifying various particular geometric structures recently studied in the literature, such as critical point equation and vacuum static equation. We show various rigidity results of Einstein-type manifolds under assumptions of several curvature conditions.

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arXiv2021

Chen's conjecture on biharmonic submanifolds in Riemannian manifolds

We study biharmonic hypersurfaces and biharmonic submanifolds in a Riemannian manifold. One of interesting problems in this direction is Chen's conjecture which says that any biharmonic submanifold in a Euclidean space is minimal. From the invariant equation for biharmonic submanifolds, we derive a fundamental identity involving the mean curvature vector field, and using this, we prove Chen's conjecture on biharmonic submanifolds in a Euclidean space. More generally, it is proved that any biharmonic submanifold in a space form of nonpositively sectional curvature is minimal. Furthermore we provide affirmative partial answers to the generalized Chen's conjecture and Balmuş-Montaldo-Oniciuc conjecture.

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arXiv2021

Closed generalized Einstein manifolds with radially flat Ricci curvature

In this paper, we show that a closed $n$-dimensional generalized $(\lambda, n+m)$-Einstein manifold of constant scalar curvature with weakly radially zero Ricci curvature is isometric to either a sphere ${\Bbb S}^n$, or a product ${\Bbb S}^{1} \times \Sigma^{n-1}$ of a circle with an $(n-1)$-dimensional Einstein manifold of positive Ricci curvature, up to finite cover and rescaling. Furthermore, if we assume $(M, g)$ has positive isotropic curvature, $M$ must be isometric to either a sphere ${\Bbb S}^n$, or a product ${\Bbb S}^{1} \times {\Bbb S}^{n-1}$ of a circle with an $(n-1)$-sphere.

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arXiv2021

V-static spaces with positive isotropic curvature

In this paper, we give a complete classification of critical metrics of the volume functional on a compact manifold $M$ with boundary $\partial M$ having positive isotropic curvature. We prove that for a pair $(f, κ)$ of a nontrivial smooth function $f: M \to {\Bbb R}$ and a nonnegative real number $κ$, if $(M, g)$ having positive isotropic curvature satisfies $$ Ddf - (Δf)g - f{\rm Ric} = κg, $$ then $(M, g)$ is isometric to a geodesic ball in ${\Bbb S}^n$ when $κ>0$, and either $M$ isometric to ${\Bbb S}^n_+$, or the product $I \times {\Bbb S}^{n-1}$, up to finite cover when $κ=0$.

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arXiv2021

Besse conjecture with positive isotropic curvature

The critical point equation arises as a critical point of the total scalar curvature functional defined on the space of constant scalar curvature metrics of a unit volume on a compact manifold. In this equation, there exists a function $f$ on the manifold that satisfies the following $$ (1+f){\rm Ric} = Ddf + \frac{nf +n-1}{n(n-1)}sg. $$ It has been conjectured that if $(g, f)$ is a solution of the critical point equation, then $g$ is Einstein and so $(M, g)$ is isometric to a standard sphere. In this paper, we show that this conjecture is true if the given Riemannian metric has positive isotropic curvature.

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arXiv2021

Vacuum Static Spaces with Positive Isotropic Curvature

In this paper, we study vacuum static spaces with positive isotropic curvature. We prove that if $(M^n, g, f)$, $n \ge 4$, is a compact vacuum static space with positive isotropic curvature, then up to finite cover, $M$ is isometric to a sphere ${\Bbb S}^n$ or the product of a circle ${\Bbb S}^1$ with an $(n-1)$-dimensional sphere ${\Bbb S}^{n-1}$.

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arXiv2019

Vacuum static spaces with vanishing of complete divergence of Bach tensor and Weyl tensor

In this paper, we study vacuum static spaces with the complete divergence of the Bach tensor and Weyl tensor. First, we prove that the vanishing of complete divergence of the Bach tensor and Weyl tensor implies the harmonicity of the metric, and we present examples in which these conditions do not imply Bach flatness. As an application, we prove the non-existence of multiple black holes in vacuum static spaces with zero scalar curvature. On the other hand, we prove the Besse conjecture under these conditions, which are weaker than harmonicity or Bach flatness. Moreover, we show a rigidity result for vacuum static spaces and find a sufficient condition for the metric to be Bach-flat.

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arXiv2018

Gap theorems on critical point equation of the total scalar curvature with divergence-free Bach tensor

On a compact $n$-dimensional manifold, it is well known that a critical metric of the total scalar curvature, restricted to the space of metrics with unit volume is Einstein. It has been conjectured that a critical metric of the total scalar curvature, restricted to the space of metrics with constant scalar curvature of unit volume, will be Einstein. This conjecture, proposed in 1987 by Besse, has not been resolved except when $M$ has harmonic curvature or the metric is Bach flat. In this paper, we prove some gap properties under divergence-free Bach tensor condition for $n\geq 5$, and a similar condition for $n=4$.

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arXiv2018

Rigidity of the total scalar curvature with divergence-free Bach tensor

On a compact $n$-dimensional manifold $M$, it is well known that a critical metric of the total scalar curvature, restricted to the space of metrics with unit volume, is Einstein. It has been conjectured that a critical metric of the total scalar curvature, restricted to the space of metrics with constant scalar curvature of unit volume, will also be Einstein. It was shown that this conjecture is true when $M$ together with a critical metric has harmonic curvature or the metric is Bach flat. In this paper, we tried to prove this conjecture with a divergence-free Bach tensor.

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arXiv2017

Besse conjecture with vanishing conditions on the Weyl tensor

On a compact $n$-dimensional manifold, it is well known that a critical metric of the total scalar curvature, restricted to the space of metrics with unit volume is Einstein. It has been conjectured that a critical metric of the total scalar curvature, restricted to the space of metrics with constant scalar curvature of unit volume, will be Einstein. This conjecture was proposed in 1987 by Besse, but has yet to be proved. In this paper, we prove the Besse conjecture with a weaker condition than harmonic curvature for $n\geq 3$.

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arXiv2016

Bach-flat h-almost gradient Ricci solitons

On an $n$-dimensional complete manifold $M$, consider an $h$-almost gradient Ricci soliton, which is a generalization of a gradient Ricci soliton. We prove that if the manifold is Bach-flat and $dh/du>0$, then the manifold $M$ is either Einstein or rigid. In particular, such a manifold has harmonic Weyl curvature. Moreover, if the dimension of $M$ is four, the metric $g$ is conformally flat.

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arXiv2016

Ridigity of Ricci Solitons with Weakly Harmonic Weyl Tensors

In this paper, we prove rigidity results on gradient shrinking Ricci solitons with weakly harmonic Weyl curvature tensors. Let $(M^n, g)$ be a compact gradient shrinking Ricci soliton satisfying ${\rm Ric}_g + Ddf = ρg$ with $ρ>0$ constant. We show that if $(M,g)$ satisfies $δ\mathcal W (\cdot, \cdot, \nabla f) = 0$, then $(M, g)$ is Einstein. Here $\mathcal W$ denotes the Weyl curvature tensor. In the case of noncompact, if $M$ is complete and satisfies the same condition, then $M$ is rigid in the sense that $M$ is given by a quotient of product of an Einstein manifold with Euclidean space. These are generalizations of the previous known results in \cite{l-r}, \cite{m-s} and \cite{p-w3}.

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arXiv2011

Variational characterizations of the total scalar curvature and eigenvalues of the Laplacian

For the dual operator $s_g'^*$ of the linearization $s_g'$ of the scalar curvature function, it is well-known that if $\ker s_g'^*\neq 0$, then $s_g$ is a non-negative constant. In particular, if the Ricci curvature is not flat, then $ {s_g}/(n-1)$ is an eigenvalue of the Laplacian of the metric $g$. In this work, some variational characterizations were performed for the space $\ker s_g'^*$. To accomplish this task, we introduce a fourth-order elliptic differential operator $\mathcal A$ and a related geometric invariant $ν$. We prove that $ν$ vanishes if and only if $\ker s_g'^* \ne 0$, and if the first eigenvalue of the Laplace operator is large compared to its scalar curvature, then $ν$ is positive and $\ker s_g'^*= 0$. Furthermore, we calculated the lower bound on $ν$ in the case of $\ker s_g'^* = 0$. We also show that if there exists a function which is $\mathcal A$-superharmonic and the Ricci curvature has a lower bound, then the first non-zero eigenvalue of the Laplace operator has an upper bound.

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arXiv2011

Total Scalar Curvature and Harmonic Curvature

On a compact n-dimensional manifold, it has been conjectured that a critical point metric of the total scalar curvature, restricted to the space of metrics with constant scalar curvature of unit volume, will be Einstein. This conjecture was proposed in 1984 by Besse, but has yet to be proved. In this paper, we prove that if the manifold with the critical point metric has harmonic curvature, then it is isometric to a standard sphere.

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arXiv1998

Surgery and the Yamabe invariant

We study the Yamabe invariant of manifolds obtained as connected sums along submanifolds of codimension greater than 2. In particular, given a compact smooth manifold M which does not admit metrics of positive scalar curvature, we prove that the Yamabe invariant of M is an upper bound for the Yamabe invariant of any manifold obtained by performing surgery in M on spheres of codimension greater than 2 .

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