arXiv subjects

Guixiang Xu

Publications and source records attributed to Guixiang Xu.

At least 19 recordsLinked to original sources

arXiv2026

Construction of two-bubble solutions for the energy-critical NLS in dimension 6

We construct pure two-bubble solutions for the energy-critical focusing nonlinear Schr\"odinger equation in space dimension $N = 6$. They are global in (at least) one time direction and approach a superposition of two stationary states, both centered at the origin. One of the bubbles develops at scale $1$, whereas the length scale of the other converges to $0$ at rate $e^{-|t|}$. The phases of the two bubbles form the right angle. Such solutions were previously constructed in dimension $N \geq 7$. The six-dimension case presents specific difficulties, as the ground state does not belong to $\dot H^{-1}$. This prevents the use of the standard method of removing linear terms in modulation equations via suitable orthogonality conditions, due to loss of coercivity of the energy functional. The main novelty of this work is the introduction of modified modulation parameters to overcome this issue; these can be viewed as an analog of a normal form transformation in the context of modulation analysis. We also establish new coercivity estimates for the linearized energy, whose positive constants depend explicitly on the choice of the orthogonality conditions.

math.AP↗

arXiv2026

Construction of two-bubble solutions for the energy-critical Hartree equation

We construct a pure two-bubble solution for the focusing, energy-critical Hartree equation in space dimension $N \geq 7$. The constructed solution is spherically symmetric, global in (at least) the negative time direction and asymptotically behaves as a superposition of two ground states (or bubbles) both centered at the origin, with the ratio of their length scales converging to $0$ and the phases of the two bubbles form the right angle. The main arguments are the modulation analysis, the bootstrap argument and the topological argument. The main novelty with respect to existing constructions of pure two-bubble solutions is the nonlocal interaction, which is more complex to analyze.

math.AP↗

arXiv2025

Global dynamics above the ground state for the energy-critical Hartree equation with radial data

Based on the concentration-compactness-rigidity argument in \cite{KenM:NLS,KenM:NLW} and the non-degeneracy of the ground state in \cite{LLTX:Nondeg,LLTX:g-Hart,LTX:Nondeg}, long time dynamics for the focusing energy-critical Hartree equation with radial data have been classified when the energy $E(u_0)\leq E(W)$ in \cite{LiMZ:crit Hart,LLTX:g-Hart,MWX:Hart,MXZ:crit Hart:f rad}, where $W$ is the ground state. In this paper, we continue the study on the dynamics of the radial solutions with the energy $E(u_0)$ at most slightly larger than that of the ground states. This is an extension of the results \cite{KriNS:NLW rad, KriNS:NLW non,NakR,NakS:NLKG,NakS:book,NakS:NLS,NakS:NLKG:non,Roy} on NLS, NLW and NLKG, which were pioneered by K. Nakanishi and W. Schlag in \cite{NakS:NLKG, NakS:book} in the study of nonlinear Klein-Gordon equation in the subcritical case. The argument is an adaptation of the works in \cite{KriNS:NLW rad, KriNS:NLW non,NakR,Roy}, the proof uses an analysis of the hyperbolic dynamics near the ground state and the variational structure far from them. The key components that allow to classify the solutions are the hyperbolic (ejection) dynamical behavior near the ground state and the one-pass lemma.

math.AP↗

arXiv2025

Global well-posedness of the defocusing nonlinear wave equation outside of a ball with radial data for $3<p<5$

We continue the study of the Dirichlet boundary value problem of nonlinear wave equation with radial data in the exterior $\Omega = \mathbb{R}^3\backslash \bar{B}(0,1)$. We combine the distorted Fourier truncation method in \cite{Bourgain98:FTM}, the global-in-time (endpoint) Strichartz estimates in \cite{XuYang:NLW} with the energy method in \cite{GallPlan03:NLW} to prove the global well-posedness of the radial solution to the defocusing, energy-subcriticial nonlinear wave equation outside of a ball in $\left(\dot H^{s}_{D}(\Omega) \cap L^{p+1}(\Omega) \right)\times \dot H^{s-1}_{D}(\Omega)$ with $1-\frac{(p+3)(1-s_c)}{4(2p-3)}<s<1$, $s_c=\frac{3}{2}-\frac{2}{p-1} $, which extends the result for the cubic nonlinearity in \cite{XuYang:NLW} to the case $3<p<5$. Except from the argument in \cite{XuYang:NLW}, another new ingredient is that we need make use of the radial Sobolev inequality to deal with the super-conformal nonlinearity in addition to the Sobolev inequality.

math.AP↗

arXiv2024

Global well-posedness of the defocusing, cubic nonlinear wave equation outside of the ball with radial data

We consider the defocusing, cubic nonlinear wave equation with zero Dirichlet boundary value in the exterior $\Omega = \mathbb{R}^3\backslash \bar{ B}(0,1)$. We make use of the distorted Fourier transform in \cite{LiSZ:NLS, Taylor:PDE:II} to establish the dispersive estimate and the global-in-time (endpoint) Strichartz estimate of the linear wave equation outside of the ball with radial data. As an application, we combine the Fourier truncation method as those in \cite{Bourgain98:FTM, GallPlan03:NLW, KenigPV00:NLW} with the energy method to show global well-posedness of radial solution to the defoucusing, cubic nonlinear wave equation outside of a ball in the Sobolev space $\left(\dot H^{s}_{D}(\Omega) \cap L^4(\Omega) \right)\times \dot H^{s-1}_{D}(\Omega)$ with $s>3/4$. To the best of the author's knowledge, it is first result about low regularity of semilinear wave equation with zero Dirichlet boundary value on the exterior domain.

math.AP↗

arXiv2023

Dynamics of radial threshold solutions for generalized energy-critical Hartree equation

In this paper, we study long time dynamics of radial threshold solutions for the focusing, generalized energy-critical Hartree equation and classify all radial threshold solutions. The main arguments are the spectral theory of the linearized operator, the modulational analysis and the concentration compactness rigidity argument developed by T. Duyckaerts and F. Merle to classify all threshold solutions for the energy critical NLS and NLW in \cite{DuyMerle:NLS:ThresholdSolution, DuyMerle:NLW:ThresholdSolution}, later by D. Li and X. Zhang in \cite{LiZh:NLS, LiZh:NLW} in higher dimensions. The new ingredient here is to solve the nondegeneracy of positive bubble solutions with nonlocal structure in $\dot H^1(\R^N)$ (i.e. the spectral assumption in \cite{MiaoWX:dynamic gHartree}) by the nondegeneracy result of positive bubble solution in $L^{\infty}(\R^N)$ in \cite{LLTX:Nondegeneracy} and the Moser iteration method in \cite{DiMeVald:book}, which is related to the spectral analysis of the linearized operator with nonlocal structure, and plays a key role in the construction of the special threshold solutions, and the classification of all threshold solutions.

math.AP↗

arXiv2023

A perturbation result for the energy critical Choquard equation in $\mathbb{R}^N$

We study the singularly perturbed nonlinear energy critical Choquard equation \begin{equation*} -{\Laplace u}\qty({x}) -α \int_{\R^N}\frac{u^p\qty(y)}{\abs{x-y}^λ}\odif{y} u^{p-1}\qty({x}) -\eps k\qty(x)u^{\frac{N+2}{N-2}}\qty(x)=0, \qquad x\in\R^N, \end{equation*} where $N\geq 3$, $0<λ 0$, we construct solutions of the form \begin{align*} u_{\eps}\qty(x)=U_{μ_{\eps},ξ_{\eps}}\qty(x)\qty(1+Ø\qty(\eps)), \end{align*} where $U_{μ_{\eps},ξ_{\eps}}$ is a positive solution of the unperturbed equation \begin{equation*} -{\Laplace u}\qty({x}) -α \int_{\R^N}\frac{u^p\qty(y)}{\abs{x-y}^λ}\odif{y}=0,\qquad x\in\R^N. \end{equation*}

math.AP↗

arXiv2023

Nondegeneracy of positive bubble solutions for generalized energy-critical Hartree equations

In this paper, we show the nondegeneracy of positive bubble solutions for generalized energy-critical Hartree equations (NLH) \begin{equation*} -{\Delta u}\sts{x} -{\bm\alpha}\sts{N,\lambda} \int_{\R^N} { \frac{ u^{p}\sts{y}}{\pabs{\,x-y\,}{\lambda}} }\diff{y}\, u^{p-1}\sts{x} =0,\quad x\in \R^N \end{equation*} where $N\geq 3$, $0<\lambda<N$, $p=\frac{2N-\lambda}{N-2}$ and ${\bm\alpha}\sts{N,\lambda}$ is a normalized constant such that $ u(x)=\left(1+|x|^2\right)^{-\frac{N-2}{2} }$ is a bubble solution of the equation \eqref{NLH}. It solves an open nondegeneracy problem in \cite{MWX:Hartree, GMYZ2022cvpde} and generalizes the partial nondegeneracy results in \cite{DY2019dcds, GWY2020na, LTX2021} to the full range $0<\lambda<N$. The key observation is that by use of the stereographic projection $\mathcal{S}$, the weighted pushforward map $\mathcal{S}_*$ is one-to-one map between the null space of the linearized operator and the spherical harmonic function subspace $\mathcal{H}_1^{N+1}$ of degree one.

math.AP↗

arXiv2022

A Mikhlin--Hörmander multiplier theorem for the partial harmonic oscillator

We prove a Mikhlin--Hörmander multiplier theorem for the partial harmonic oscillator $H_{\textup{par}}=-\pa_ρ^2-Δ_x+|x|^2$ for $(ρ, x)\in\R\times\R^d$ by using the Littlewood--Paley $g$ and $g^\ast$ functions and the associated heat kernel estimate. The multiplier we have investigated is defined on $\mathbb R \times \mathbb N$.

math.AP↗

arXiv2022

Riesz transforms and Sobolev spaces associated to the partial harmonic oscillator

In this paper, our goal is to establish the Sobolev space associated to the partial harmonic oscillator. Based on its heat kernel estimate, we firstly give the definition of the fractional powers of the partial harmonic oscillator $$\AH=-\partial_{\rho}^2-\Delta_x+|x|^2,$$ and show that its negative powers are well defined on $L^p(\mathbb R^{d+1})$ for $p\in [1,\infty]$. We then define associated Riesz transforms and show that they are bounded on classical Sobolev spaces by the calculus of symbols. Secondly, by a factorization of the operator $\AH$, we define two families of Sobolev spaces with positive integer indices, and show the equivalence between them by the boundedness of Riesz transforms. Moreover, the adapted symbolic calculus also implies the boundedness of Riesz type transforms on the Sobolev spaces associated to the partial harmonic oscillator $\AH$. Lastly, as applications of our results, we obtain the revised Hardy-Littlewood-Sobolev inequality, the Gagliardo-Nirenberg-Sobolev inequality, and Hardy's inequality in the potential space $L_{\AH}^{\alpha, p}$.

math.AP↗

arXiv2021

Nondegeneracy of the positive solutions for critical nonlinear Hartree equation in $\R^6$

We prove that any positive solution for the critical nonlinear Hartree equation $$-\Laplacian\fct{u}{x} -\int_{\R^6} \frac{\abs{\fct{u}{y}}^2 }{ \abs{x-y}^4 }\odif{y} \,\fct{u}{x}=0,\qtq{} x\in\R^6.$$ is nondegenerate. Firstly, in terms of spherical harmonics, we show that the corresponding linear operator can be decomposed into a series of one dimensional linear operators. Secondly, by making use of the Perron-Frobenius property, we show that the kernel of each one dimensional linear operator is finite. Finally, we show that the kernel of the corresponding linear operator is the direct sum of the kernel of all one dimensional linear operators.

math.AP↗

arXiv2021

Minimal mass blow-up solutions for the $L^2$-critical NLS with the Delta potential for radial data in one dimension

We consider the $L^2$-critical nonlinear Schrödinger equation (NLS) with the delta potential $$i\partial_tu +\partial^2_x u + μδu +|u|^{4}u=0, \, \, t\in \R, \, x\in \R , $$ where $ μ\in \R$, and $δ$ is the Dirac delta distribution at $x=0$. Local well-posedness theory together with sharp Gagliardo-Nirenberg inequality and the conservation laws of mass and energy implies that the solution with mass less than $\|Q\|_{2}$ is global existence in $H^1(\R)$, where $Q$ is the ground state of the $L^2$-critical NLS without the delta potential (i.e. $μ=0$). We are interested in the dynamics of the solution with threshold mass $\|u_0\|_{2}=\|Q\|_{2}$ in $H^1(\R)$. First, for the case $μ=0$, such blow-up solution exists due to the pseudo-conformal symmetry of the equation, and is unique up to the symmetries of the equation in $H^1(\R)$ from \cite{Me93:NLS:mini sol} (see also \cite{HmKe05:NLS:mini blp}), and recently in $L^2(\R)$ from \cite{Dod:NLS:L2thrh1}. Second, for the case $μ<0$, simple variational argument with the conservation laws of mass and energy implies that radial solutions with threshold mass exist globally in $H^1(\R)$. Last, for the case $μ>0$, we show the existence of radial threshold solutions with blow-up speed determined by the sign (i.e. $μ>0$) of the delta potential perturbation since the refined blow-up profile to the rescaled equation is stable in a precise sense. The key ingredients here including the Energy-Morawetz argument and compactness method as well as the modulation analysis are close to the original one in \cite{RaS11:NLS:mini sol} (see also \cite{KrLR13:HalfW:nondis, LeMR:CNLS:blp, Mart05:Kdv:N sol, MaP17:BO:mini sol, MeRS14:NLS:blp}).

math.AP↗

arXiv2021

A Limiting absorption principle for high-order Schrödinger operators in critical spaces

In this paper, we prove a limiting absorption principle for high-order Schrödinger operators with a large class of potentials which generalize some results by A. Ionescu and W. Schlag. Our main idea is to handle the boundary operators by the restriction theorem of Fourier transform. Two key tools we use in this paper are the Stein--Tomas theorem in Lorentz spaces and a sharp trace lemma given by S. Agmon and L. Hörmander

math.AP↗

arXiv2020

Microscopic conservation laws for the derivative Nonlinear Schrödinger equation

Compared with macroscopic conservation law for the solution of the derivative nonlinear Schrödingger equation (DNLS) with small mass in \cite{KlausS:DNLS}, we show the corresponding microscopic conservation laws for the Schwartz solutions of DNLS with small mass. The new ingredient is to make use of the logarithmic perturbation determinant introduced in \cite{Rybkin:KdV:Cons Law, Simon:Trace} to show one-parameter family of microscopic conservation laws of the $A(κ)$ flow and the DNLS flow, which is motivated by \cite{HKV:NLS,KV:KdV:AnnMath,KVZ:KdV:GAFA}.

math.AP↗

arXiv2020

Entire sign-changing solutions to the fractional critical Schr{ö}dinger equation

We consider the fractional critical Schr{ö}dinger equation (FCSE) \begin{align*} \slaplace{u}-\abs{u}^{2^{\ast}_{s}-2}u=0, \end{align*} where $u \in \dot H^s( \R^N)$, $N\geq 2$, $0<s<1$ and $2^{\ast}_{s}=\frac{2N}{N-2s}$. By virtue of the mini-max theory and the concentration compactness principle with the equivariant group action, we obtain the new type of non-radial, sign-changing solutions of (FCSE) in the energy space $\dot H^s(\R^N)$. The key component is that we use the equivariant group to partion $\dot H^s(\R^N)$ into several connected components, then combine the concentration compactness argument to show the compactness property of Palais-Smale sequences in each component and obtain many solutions of (FCSE) in $\dot H^s(\R^N)$. Both the solutions and the argument here are different from those by Garrido, Musso in \cite{GM2016pjm} and by Abreu, Barbosa and Ramirez in \cite{ABR2019arxiv}.

math.AP↗

arXiv2019

Instability of the solitary waves for the 1d NLS with an attrictive delta potential in the degenerate case

In this paper, we show the orbital instability of the solitary waves $Q_Ωe^{iΩt}$ of the 1d NLS with an attractive delta potential ($γ>0$) \begin{equation*} ıu_t+u_{xx}+γδu+\abs{u}^{p-1}u=0, \; p>5, \end{equation*} where $Ω=Ω(p,γ)>\frac{γ^2}{4}$ is the critical oscillation number and determined by \begin{equation*} \frac{p-5}{p-1} \int_{ \arctanh\sts{ \fracγ{2\sqrtΩ} } }^{+\infty} \sech^{\frac{4}{p-1}}\sts{y}\d y = { \fracγ{ 2\sqrtΩ } }\sts{ 1-\frac{γ^2}{4Ω} }^{-\frac{p-3}{p-1}} \Longleftrightarrow \mathbf{d}''(Ω) =0. \end{equation*} The classical convex method and Grillakis-Shatah-Strauss's stability approach in \cite{A2009Stab, GSS1987JFA1} don't work in this degenerate case, and the argument here is motivated by those in \cite{CP2003CPAM, MM2001GAFA, M2012JFA, MTX2018, O2011JFA}. The main ingredients are to construct the unstable second order approximation near the solitary wave $Q_Ωe^{iΩt}$ on the level set $\Mcal(Q_Ω)$ accoding to the degenerate structure of the Hamiltonian and to construct the refined Virial identity to show the orbital instability of the solitary waves $Q_Ωe^{iΩt}$ in the energy space. Our result is the complement of the results in \cite{FOO2008AIHP} in the degenerate case.

math.AP↗

arXiv2018

Global well-posedness for the defocusing Hartree equation with radial data in $\mathbb R^4$

By $I$-method, the interaction Morawetz estimate, long time Strichartz estimate and local smoothing effect of Schrödinger operator, we show global well-posedness and scattering for the defocusing Hartree equation $$\left\{ \begin{array}{ll} iu_t + Δu &=F(u), \quad (t,x) \in \mathbb{R} \times \mathbb{R}^4 u(0) \\ &=u_0(x)\in H^s(\mathbb{R}^4), \end{array} \right. $$ where $F(u)= (V* |u|^2) u$, and $V(x)=|x|^{-γ}$, $3< γ<4$, with radial data in $H^{s}(\mathbb{R}^4)$ for $s>s_c:=γ/2-1$. It is a sharp global result except of the critical case $s=s_c$, which is a very difficult open problem.

math.AP↗

arXiv2018

Instability of the solitary waves for the generalized derivative nonlinear Schrödinger equation in the degenerate case

In this paper, we develop the modulation analysis, the perturbation argument and the Virial identity similar as those in \cite{MartelM:Instab:gKdV} to show the orbital instability of the solitary waves $\Q\sts{x-ct}\e^{ıωt}$ of the generalized derivative nonlinear Schrödinger equation (gDNLS) in the degenerate case $c=2z_0\sqrtω$, where $z_0=z_0\stsσ $ is the unique zero point of $F\sts{z;~σ}$ in $\sts{-1, ~ 1}$. The new ingredients in the proof are the refined modulation decomposition of the solution near $\Q$ according to the spectrum property of the linearized operator $\Scal_{ω, c}"\sts{\Q}$ and the refined construction of the Virial identity in the degenerate case. Our argument is qualitative, and we improve the result in \cite{Fukaya2017}.

math.AP↗