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Seunghyeok Kim

Publications and source records attributed to Seunghyeok Kim.

At least 19 recordsLinked to original sources
arXiv2026

Uncountably many non-rotationally symmetric type II ancient Yamabe flows on the sphere

For every $n \ge 3$, we construct uncountably many families of type II ancient solutions to the Yamabe flow on the unit round $n$-sphere $\Ss^n$. These families are pairwise distinct up to conformal equivalence, and no member is conformally equivalent to a rotationally symmetric solution. At every negative time, the Ricci curvature tensor of each solution is indefinite at some point. Moreover, the associated backward limit space is a wedge sum of finitely many isometric copies of $\Ss^n$. These examples show that the collection of ancient Yamabe flows on $\Ss^n$ has a much richer structure than suggested by two natural comparison problems: the compact ancient Ricci flows on $\Ss^2$, all of which are known to be rotationally symmetric, and the elliptic Yamabe equation on $\R^n$, whose positive entire solutions are only the standard bubbles. The construction uses a non-radial inner--outer gluing scheme. After stereographic projection, we reformulate the flow as a conformally invariant parabolic problem on $\R^n$. By exploiting Kelvin invariance and switching between the Euclidean and spherical formulations as needed, we control the non-radial modes directly without reducing the problem to one space dimension. Weighted Hölder estimates provide the pointwise control needed to establish the Type II behavior, the Ricci-sign property, conformal inequivalence, and the description of the backward limits in a straightforward manner.

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arXiv2025

Compactness and non-compactness theorems of the fourth- and sixth-order constant $Q$-curvature problems

We provide a complete resolution to the question of compactness for the full solution sets of the fourth-order and sixth-order constant $Q$-curvature problems on smooth closed Riemannian manifolds not conformally diffeomorphic to the standard unit $n$-sphere, provided the associated conformally covariant differential operator has a positive Green's function. Firstly, we prove that the solution set of the fourth-order constant $Q$-curvature problem is $C^4$-compact in dimensions $5 \le n \le 24$. For $n \ge 25$, an example of an $L^{\infty}$-unbounded sequence of solutions has been known for over a decade (Wei and Zhao). Additionally, the compactness result for $5 \le n \le 9$ was established by Li and Xiong. Secondly, we demonstrate that the solution set of the sixth-order constant $Q$-curvature problem is $C^6$-compact in dimensions $7 \le n \le 26$, whereas a blow-up example exists for $n \ge 27$. Our main observation is that the linearized equations associated with both $Q$-curvature problems can be transformed into overdetermined linear systems, which admit nontrivial solutions due to unexpected algebraic structures of the Paneitz operator and the sixth-order GJMS operator. This key insight not only plays a crucial role in deducing the compactness result for high-dimensional manifolds, but also reveals an elegant hierarchical pattern with respect to the order of the conformally covariant operators, suggesting the possibility of a unified theory of the compactness of the constant $Q$-curvature problems of all admissible even integer orders.

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arXiv2025

Sign-changing solutions for critical Hamiltonian systems in $\mathbb{R}^N$

We build infinitely many geometrically distinct non-radial sign-changing solutions for the Hamiltonian-type elliptic systems $$ -Δu =|v|^{p-1}v\ \hbox{in}\ \mathbb{R}^N,\ -Δv =|u|^{q-1}u\ \hbox{in}\ \mathbb{R}^N,$$ where the exponents $(p,q)$ satisfy $p,q>1$ and belong to the critical hyperbola $$\frac1{p+1}+\frac1{q+1} =\frac {N-2}N.$$ To establish this result, we introduce several new ideas and strategies that are both robust and potentially applicable to other critical problems lacking the Kelvin invariance.

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arXiv2025

Sharp quantitative stability estimates for the Brezis-Nirenberg problem

We study the quantitative stability for the classical Brezis-Nirenberg problem associated with the critical Sobolev embedding $H^1_0(Ω) \hookrightarrow L^{\frac{2n}{n-2}}(Ω)$ in a smooth bounded domain $Ω\subset \mathbb{R}^n$ ($n \geq 3$). To the best of our knowledge, this work presents the first quantitative stability result for the Sobolev inequality on bounded domains. A key discovery is the emergence of unexpected stability exponents in our estimates, which arise from the intricate interaction among the nonnegative solution $u_0$ and the linear term $λu$ of the Brezis--Nirenberg equation, bubble formation, and the boundary effect of the domain $Ω$. One of the main challenges is to capture the boundary effect quantitatively, a feature that fundamentally distinguishes our setting from the Euclidean case treated in \cite{CFM, FG, DSW} and the smooth closed manifold case studied in \cite{CK}. In addressing a variety of difficulties, our proof refines and streamlines several arguments from the existing literature while also resolving new analytical challenges specific to our setting.

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arXiv2024

Weighted isoperimetric ratios and extension problems for fractional conformal Laplacians

We investigate a novel connection between the weighted isoperimetric problems and the weighted Poisson integrals of the extension problems for nonlocal elliptic operators. We first derive sharp inequalities for the weighted Poisson integrals associated with degenerate elliptic equations on the half-space and the unit ball, and classify their extremizers. The equations arise from the Caffarelli-Silvestre extension for the fractional Laplacian on the Euclidean space and its conformal transformation via the Möbius transformation. We next interpret the above sharp inequalities in a conformal geometric viewpoint. For this aim, we formulate a variational problem involving a weighted isoperimetric ratio on a smooth metric measure space induced by a conformally compact Einstein (CCE) manifold. Then, we prove that the variational problem is closely linked to the Chang-González extension for a fractional conformal Laplacian on the conformal infinity of the CCE manifold, and is reduced to the sharp inequality if the CCE manifold is either the Poincaré half-space or ball model. We also find a criterion that ensures the existence of a smooth extremizer of the variational problem, and present a relevant conjecture.

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arXiv2024

Sharp quantitative stability of the Yamabe problem

Given a smooth closed Riemannian manifold $(M,g)$ of dimension $N \ge 3$, we derive sharp quantitative stability estimates for nonnegative functions near the solution set of the Yamabe problem on $(M,g)$. The seminal work of Struwe (1984) \cite{S} states that if $Γ(u) := \|Δ_g u - \frac{N-2}{4(N-1)} R_g u + u^{\frac{N+2}{N-2}}\|_{H^{-1}(M)} \to 0$, then $\|u-(u_0+\sum_{i=1}^ν \mathcal{V}_i)\|_{H^1(M)} \to 0$ where $u_0$ is a solution to the Yamabe problem on $(M,g)$, $ν\in \mathbb{N} \cup \{0\}$, and $\mathcal{V}_i$ is a bubble-like function. If $M$ is the round sphere $\mathbb{S}^N$, then $u_0 \equiv 0$ and a natural candidate of $\mathcal{V}_i$ is a bubble itself. If $M$ is not conformally equivalent to $\mathbb{S}^N$, then either $u_0 > 0$ or $u_0 \equiv 0$, there is no canonical choice of $\mathcal{V}_i$, and so a careful selection of $\mathcal{V}_i$ must be made to attain optimal estimates. For $3 \le N \le 5$, we construct suitable $\mathcal{V}_i$'s and then establish the inequality $\|u-(u_0+\sum_{i=1}^ν \mathcal{V}_i)\|_{H^1(M)}$ $ \le Cζ(Γ(u))$ where $C > 0$ and $ζ(t) = t$, consistent with the result of Figalli and Glaudo (2020) \cite{FG} on $\mathbb{S}^N$. In the case of $N \ge 6$, we investigate the single-bubbling phenomenon $(ν= 1)$ on generic Riemannian manifolds $(M,g)$, proving that $ζ(t)$ is determined by $N$, $u_0$, and $g$, and can be much larger than $t$. This exhibits a striking difference from the result of Ciraolo, Figalli, and Maggi (2018) \cite{CFM} on $\mathbb{S}^N$. All of the estimates presented herein are optimal.

math.AP↗
arXiv2022

Coron's problem for the critical Lane-Emden system

In this paper, we address the solvability of the critical Lane-Emden system \[\begin{cases} -Δu=|v|^{p-1}v &\mbox{in } Ω_ε,\\ -Δv=|u|^{q-1}u &\mbox{in } Ω_ε,\\ u=v=0 &\mbox{on } \partial Ω_ε, \end{cases}\] where $N \ge 4$, $p \in (1,\frac{N-1}{N-2})$, $\frac{1}{p+1} + \frac{1}{q+1}=\frac{N-2}{N}$, and $Ω_ε$ is a smooth bounded domain with a small hole of radius $ε> 0$. We prove that the system admits a family of positive solutions that concentrate around the center of the hole as $ε\to 0$, obtaining a concrete qualitative description of the solutions as well. To the best of our knowledge, this is the first existence result for the critical Lane-Emden system on a bounded domain, while the non-existence result on star-shaped bounded domains has been known since the early 1990s due to Mitidieri (1993) [30] and van der Vorst (1991) [36].

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arXiv2022

Asymptotic analysis on positive solutions of the Lane-Emden system with nearly critical exponents

We concern a family $\{(u_{\varepsilon},v_{\varepsilon})\}_{\varepsilon > 0}$ of solutions of the Lane-Emden system on a smooth bounded convex domain $Ω$ in $\mathbb{R}^N$ \[\begin{cases} -Δu_{\varepsilon} = v_{\varepsilon}^p &\text{in } Ω,\\ -Δv_{\varepsilon} = u_{\varepsilon}^{q_{\varepsilon}} &\text{in } Ω,\\ u_{\varepsilon},\, v_{\varepsilon} > 0 &\text{in } Ω,\\ u_{\varepsilon} = v_{\varepsilon} =0 &\text{on } \partialΩ\end{cases}\] for $N \ge 4$, $\max\{1,\frac{3}{N-2}\} < p < q_{\varepsilon}$ and small \[\varepsilon := \frac{N}{p+1} + \frac{N}{q_{\varepsilon}+1} - (N-2) > 0.\] This system appears as the extremal equation of the Sobolev embedding $W^{2,(p+1)/p}(Ω) \hookrightarrow L^{q_{\varepsilon}+1}(Ω)$, and is also closely related to the Calderón-Zygmund estimate. Under the a natural energy condition \[\sup_{\varepsilon > 0} \left(\|u_{\varepsilon}\|_{W^{2,{p+1 \over p}}(Ω)} + \|v_{\varepsilon}\|_{W^{2,{q_{\varepsilon}+1 \over q_{\varepsilon}}}(Ω)}\right) < \infty,\] we prove that the multiple bubbling phenomena may arise for the family $\{(u_{\varepsilon},v_{\varepsilon})\}_{\varepsilon > 0}$, and establish a detailed qualitative and quantitative description. If $p < \frac{N}{N-2}$, the nonlinear structure of the system makes the interaction between bubbles so strong, so the determination process of the blow-up rates and locations is completely different from that of the classical Lane-Emden equation. If $p \ge \frac{N}{N-2}$, the blow-up scenario is relatively close to (but not the same as) that of the classical Lane-Emden equation, and only one-bubble solutions can exist. Even in the latter case, the standard approach does not work well, which forces us to devise a new method. Using our analysis, we also deduce a general existence theorem valid on any smooth bounded domains.

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arXiv2021

Infinite-time blowing-up solutions to small perturbations of the Yamabe flow

Under the validity of the positive mass theorem, the Yamabe flow on a smooth compact Riemannian manifold of dimension $N \ge 3$ is known to exist for all time $t$ and converges to a solution to the Yamabe problem as $t \to \infty$. We prove that if a suitable perturbation, which may be smooth and arbitrarily small, is imposed on the Yamabe flow on any given Riemannian manifold $M$ of dimension $N \ge 5$, the resulting flow may blow up at multiple points on $M$ in the infinite time. Our proof is constructive, and indeed we construct such a flow by using solutions of the Yamabe problem on the unit sphere $\mathbb{S}^N$ as blow-up profiles. We also examine the stability of the blow-up phenomena under a negativity condition on the Ricci curvature at blow-up points.

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arXiv2020

Multiple blowing-up solutions to critical elliptic systems in bounded domains

We construct families of blowing-up solutions to elliptic systems on smooth bounded domains in the Euclidean space, which are variants of the critical Lane-Emden system and analogous to the Brezis-Nirenberg problem. We find a function which governs blowing-up points and rates, observing that it reflects the strong nonlinear characteristic of the system. By using it, we also prove that a single blowing-up solution exists in general domains, and construct examples of contractible domains where multiple blowing-up solutions are allowed to exist. We believe that a variety of new ideas and arguments developed here will help to analyze blowing-up phenomena in related Hamiltonian-type systems.

math.AP↗
arXiv2019

Compactness of scalar-flat conformal metrics on low-dimensional manifolds with constant mean curvature on boundary

We concern $C^2$-compactness of the solution set of the boundary Yamabe problem on smooth compact Riemannian manifolds with boundary provided that their dimensions are $4$, $5$ or $6$. By conducting a quantitative analysis of a linear equation associated with the problem, we prove that the trace-free second fundamental form must vanish at possible blow-up points of a sequence of blowing-up solutions. Applying this result and the positive mass theorem, we deduce the $C^2$-compactness for all $4$-manifolds (which may be non-umbilic). For the $5$-dimensional case, we also establish that a sum of the second-order derivatives of the trace-free second fundamental form is non-negative at possible blow-up points. We essentially use this fact to obtain the $C^2$-compactness for all $5$-manifolds. Finally, we show that the $C^2$-compactness on $6$-manifolds is true if the trace-free second fundamental form on the boundary never vanishes.

math.AP↗
arXiv2019

Non-degeneracy for the critical Lane-Emden system

We prove the non-degeneracy for the critical Lane--Emden system $$ -ΔU = V^p,\quad -ΔV = U^q,\quad U, V > 0 \quad \text{in } \mathbb{R}^N $$ for all $N \ge 3$ and $p,q > 0$ such that $\frac{1}{p+1} + \frac{1}{q+1} = \frac{N-2}{N}$. We show that all solutions to the linearized system around a ground state must arise from the symmetries of the critical Lane-Emden system provided that they belong to the corresponding energy space or they decay to 0 uniformly as the point tends to infinity.

math.AP↗
arXiv2019

A perturbative approach to non-degeneracy of the Lane-Emden system

We consider ground state solutions of the critical Lane-Emden system \[\begin{cases} -Δu = v^p &\text{in } \mathbb{R}^n,\\ -Δv = u^q &\text{in } \mathbb{R}^n,\\ u,v >0\ &\text{in } \mathbb{R}^n, \end{cases}\] where $n \ge 3$ and $p,q>0$ and $(p,q)$ belongs to the critical hyperbola $\frac{1}{p+1} + \frac{1}{q+1} = \frac{n-2}{n}.$ We prove that they are non-degenerate when either $(p,q)$ is close to $(1,{n+4\over n-4})$ (if $n\ge5$) or $(p,q)$ is close to $({n+2\over n-2},{n+2\over n-2})$ (if $n\ge3$).

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arXiv2018

Conformal metrics with prescribed fractional scalar curvature on conformal infinities with positive fractional Yamabe constants

Let $(X, g^+)$ be an asymptotically hyperbolic manifold and $(M, [\hat{h}])$ its conformal infinity. Our primary aim in this paper is to introduce the prescribed fractional scalar curvature problem on $M$ and provide solutions under various geometric conditions on $X$ and $M$. We also obtain the existence results for the fractional Yamabe problem in the endpoint case, e.g., $n = 3$, $γ= 1/2$ and $M$ is non-umbilic, etc. Every solution we find turns out to be smooth on $M$.

math.AP↗
arXiv2018

A compactness theorem of the fractional Yamabe problem, Part I: The non-umbilic conformal infinity

Assume that $(X, g^+)$ is an asymptotically hyperbolic manifold, $(M, [\bar{h}])$ is its conformal infinity, $ρ$ is the geodesic boundary defining function associated to $\bar{h}$ and $\bar{g} = ρ^2 g^+$. For any $γ\in (0,1)$, we prove that the solution set of the $γ$-Yamabe problem on $M$ is compact in $C^2(M)$ provided that convergence of the scalar curvature $R[g^+]$ of $(X, g^+)$ to $-n(n+1)$ is sufficiently fast as $ρ$ tends to 0 and the second fundamental form on $M$ never vanishes. Since most of the arguments in blow-up analysis performed here is irrelevant to the geometric assumption imposed on $X$, our proof also provides a general scheme toward other possible compactness theorems for the fractional Yamabe problem.

math.AP↗
arXiv2018

Asymptotic behavior of least energy solutions to the Lane-Emden system near the critical hyperbola

The Lane-Emden system is written as \begin{equation*} \begin{cases} -Δu = v^p &\text{in } Ω,\\ -Δv = u^q &\text{in } Ω,\\ u, v > 0 &\text{in } Ω,\\ u = v = 0 &\text{on } \partial Ω\end{cases} \end{equation*} where $Ω$ is a smooth bounded domain in the Euclidean space $\mathbb{R}^n$ for $n \ge 3$ and $0< p< q <\infty$. The asymptotic behavior of least energy solutions near the critical hyperbola was studied by Guerra \cite{G} when $p \geq 1$ and the domain is convex. In this paper, we cover all the remaining cases $p < 1$ and extend the results to any smooth bounded domain.

math.AP↗
arXiv2016

Minimal energy solutions to the fractional Lane-Emden system, I: Existence and singularity formation

This is the first of two papers which study asymptotic behavior of minimal energy solutions to the fractional Lane-Emden system in a smooth bounded domain $Ω$ \[(-Δ)^s u = v^p, \quad (-Δ)^s v = u^q \text{ in } Ω\quad \text{and} \quad u = v = 0 \text{ on } \pa Ω\quad \text{for } 0 < s < 1\] under the assumption that the subcritical pair $(p,q)$ approaches to the critical Sobolev hyperbola. If $p = 1$, the above problem is reduced to the subcritical higher-order fractional Lane-Emden equation with the Navier boundary condition \[(-Δ)^s u = u^{\frac{n+2s}{n-2s}-\ep} \text{ in } Ω\quad \text{and} \quad u = (-Δ)^{s \over 2} u = 0 \quad \text{for } 1 < s < 2.\] The main objective of this paper is to deduce the existence of minimal energy solutions, and to examine their (normalized) pointwise limits provided that $Ω$ is convex. As a by-product of our study, a new approach for the existence of an extremal function for the Hardy-Littlewood-Sobolev inequality is provided.

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