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Linjie Song

Publications and source records attributed to Linjie Song.

At least 19 recordsLinked to original sources
arXiv2026

Scalar-induced gravitational waves from inflation with symmetry breaking

We investigate scalar-induced gravitational waves (SIGWs) in an inflationary model with symmetry breaking, in which charged scalar fields are coupled to an isotropic triplet of Abelian gauge fields through a kinetic function. Such SIGWs can be enhanced to a detectable level when the mixing between the inflaton and gauge-field perturbations is sufficiently large. We find that the longitudinal mode and the charge-dependent mixing between perturbations become relevant only when the gauge-field excitation occurs sufficiently late during inflation. In this regime, the corresponding SIGWs are shifted to ultra-high frequencies, typically can around the GHz band. We show that the parameters characterizing the effects of the longitudinal mode and charge-dependent mixing affect the signal in qualitatively different ways. This provides characteristic signatures for distinguishing the neutral case from the charged one through the frequency profile of the stochastic gravitational-wave background.

astro-ph.CO↗
arXiv2026

Exact number of positive solutions and existence of sign-changing solutions with prescribed mass for NLS on bounded domains

Given $\mu > 0$, we study the elliptic problem: \begin{align*} \text{ find } (u,\lambda) \in H_0^1(\Omega) \times \mathbb{R} \text{ such that } -\Delta u + \lambda u = |u|^{p-2}u \text{ in } \Omega \text{ and } \int_\Omega|u|^2dx = \mu, \end{align*} where $\Omega \subset \mathbb{R}^N$ is a bounded domain and $p > 2$ is Sobolev-subcritical. When $p$ is $L^2$-subcritical, i.e. $2 < p < 2 + 4/N$, we show that the problem admits infinitely many sign-changing solutions whose energies are unbounded for every fixed $\mu > 0$. Moreover, we give the limit behavior for both the parameter $\lambda$ and the energy of the solutions as $\mu \to 0^+$ and $\mu \to +\infty$ respectively. Such a multiplicity result also holds when $p$ is $L^2$-critical, i.e. $p = 2 + 4/N$, for each small $\mu > 0$, and we describe precisely what happen when $\mu \to 0^+$. In the $L^2$-supercritical case, i.e. $2+4/N < p < 2^*$, we find as many sign-changing solutions as we want at the expense of possibly reducing the mass $\mu$. As $\mu$ tends to $0$, the energy of these solutions goes to $0$ and the limit of the parameter $\lambda$ is a Dirichlet eigenvalue of $-\Delta$ on $\Omega$ multiplying $-1$. When $\Omega = B_1$, the unitary ball, and the nonlinear term is $\tau |u|^{p-2}u$ with $\tau \in [1/2,1]$ fixed, in the $L^2$-supercritical regime, we prove that the problem admits exactly two positive solutions for small $\mu > 0$ and how small $\mu > 0$ must be does not depend on the value of $\tau$. Moreover, sending $\mu$ to $0$ we get that the energy of one positive solution tends to $0$ and the parameter tends to $-\lambda_1(B_1)$, where $\lambda_1(B_1)$ is the first Dirichlet eigenvalue of $-\Delta $ on the unit ball $B_1$, while both the energy of the other positive solution and the parameter $\lambda$ go to infinity uniformly with respect to $\tau$.

math.AP↗
arXiv2025

Multiple sign-changing and semi-nodal normalized solutions for a Gross-Pitaevskii type system on bounded domains: the $L^2$-supercritical case

In this paper we investigate the existence of multiple sign-changing and semi-nodal normalized solutions for an $m$-coupled elliptic system of the Gross-Pitaevskii type: \begin{equation} \left\{ \begin{aligned} &-\Delta u_j + \lambda_j u_j = \sum_{k=1 }^m\beta_{kj} u_k^2 u_j, \quad u_j \in H_0^1(\Omega), &\int_\Omega u_j^2dx = c_j, \quad j = 1,2,\cdots,m. \end{aligned} \right. \end{equation} Here, $\Omega \subset \mathbb{R}^N$ ($N = 3,4$) is a bounded domain. The constants $\beta_{kj} \neq 0$ and $c_j > 0$ are prescribed constants, while $\lambda_1, \cdots, \lambda_m$ are unknown and appear as Lagrange multipliers. This is the first result in the literature on the existence and multiplicity of sign-changing and semi-nodal normalized solutions of couple Schr\"odinger system in all regimes of $\beta_{kj}$. The main tool which we use is a new skill of vector linking and this article attempts for the first time to use linking method to search for solutions of a coupled system. Particularly, to obtain semi-nodal normalized solutions, we introduce partial vector linking which is new up to our knowledge. Moreover, by investigating the limit process as $\vec{c}=(c_1,\ldots,c_m) \to \vec{0}$ we obtain some bifurcation results. Note that when $N=4$, the system is of Sobolev critical.

math.AP↗
arXiv2025

Multiplicity result for a mass supercritical NLS with a partial confinement

We consider an NLS equation in $\mathbb{R}^3$ with partial confinement and mass supercritical nonlinearity. In Bellazzini, Boussaid, Jeanjean and Visciglia (Comm. Math. Phys. 353, 2017, 229-251) for such a problem, a solution with a prescribed $L^2$ norm was obtained, as a local minima, and the existence of a second solution, at a mountain pass energy level, was proposed as an open problem. We give here a positive, non-perturbative, answer to this problem. Our solution is obtained as a limit of a sequence of solutions of corresponding problems on bounded domains of $\mathbb{R}^3$. The symmetry of solutions on bounded domains is used centrally in the convergence process.

math.AP↗
arXiv2025

Sign-changing prescribed mass solutions for $L^2$-supercritical NLS on compact metric graphs

This paper is devoted to the existence of multiple sign-changing solutions of prescribed mass for a mass-supercritical nonlinear Schr\"odinger equation set on a compact metric graph. In particular, we obtain, in the supercritical mass regime, the first multiplicity result for prescribed mass solutions on compact metric graphs. As a byproduct, we prove that any eigenvalue of the associated linear operator is a bifurcation point. Our approach relies on the introduction a new kind of link and on the use of gradient flow techniques on a constraint. It can be transposed to other problems posed on a bounded domain.

math.AP↗
arXiv2024

Two Positive Normalized Solutions on Star-shaped Bounded Domains to the Br\'ezis-Nirenberg Problem, I: Existence

We develop a new framework to prove the existence of two positive solutions with prescribed mass on star-shaped bounded domains: one is the normalized ground state and another is of M-P type. We merely address the Sobolev critical cases since the Sobolev subcritical ones can be addressed by following similar arguments and are easier. Our framework is based on some important observations, that, to the best of our knowledge, have not appeared in previous literatures. Using these observations, we firstly establish the existence of a normalized ground state solution, whose existence is unknown so for. Then we use some novel ideas to obtain the second positive normalized solution, which is of M-P type. It seems to be the first time in the literatures to get two positive solutions under our settings, even in the Sobolev subcritical cases. We further remark that our framework is applicable to many other equations.

math.AP↗
arXiv2024

Uniqueness of Positive Solutions for Fractional Schr\"{o}dinger Equations with General Nonlinearities

In 2013, R.L. Frank and E. Lenzmann [R.L. Frank, E. Lenzmann, Uniqueness of non-linear ground states for fractional Laplacians in $\mathbb{R}$, Acta Math. 210 (2) (2013) 261-318] study the following problem: \begin{align*} (-\Delta)^su + u = u^{p-1} \quad \text{in } \mathbb{R}^N, \end{align*} where $s \in (0,1)$, $N = 1$, $p \in (2,2_s^*)$, and $2_s^*$ is the critical fractional Sobolev exponent. They proved that the ground state is unique (up to translations). Then in 2016, they, together with L. Silvestre [R.L. Frank, E. Lenzmann, L. Silvestre, Uniqueness of radial solutions for the fractional Laplacian, Comm. Pure Appl. Math. 69 (9) (2016) 1671-1726] showed similar uniqueness results for high dimensions ($N \geqslant 2$), in which they proposed a challenging open problem to extend their results about non-degeneracy and uniqueness of ground states to nonlinearities $f(u)$ beyond the pure-power case. To the best of our knowledge, this question is still unresolved so far. In this paper, we aim to give a full affirmative answer to this open issue for a large class of convex nonlinearities. In fact, we prove the uniqueness of the positive solution.

math.AP↗
arXiv2023

Existence and non-existence results to a mixed Schrodinger system in a plane

This article focuses on the existence and non-existence of solutions for the following system of local and nonlocal type \begin{equation*} \left\{ \begin{aligned} -\partial_{xx}u + (-\Delta)_{y}^{s_{1}} u + u - u^{2_{s_{1}}^{}-1} = \kappa \alpha h(x,y) u^{\alpha-1}v^{\beta} & \quad \mbox{in} ~ \mathbb{R}^{2}, -\partial_{xx}v + (-\Delta)_{y}^{s_{2}} v + v- v^{2_{s_{2}}^{}-1} = \kappa \beta h(x,y) u^{\alpha}v^{\beta-1} & \quad \mbox{in} ~ \mathbb{R}^{2}, u,v ~ \geq ~0 \quad \mbox{in} ~ \mathbb{R}^{2}, \end{aligned} \right. \end{equation*} where $s_{1},s_{2} \in (0,1),~\alpha,\beta>1,~\alpha+\beta \leq \min \{ 2_{s_{1}}^{},2_{s_{2}}^{}\}$, and $2_{s_i}^{} = \frac{2(1+s_i)}{1-s_i}, i=1,2$. The existence of a ground state solution entirely depends on the behaviour of the parameter $\kappa>0$ and on the function $h$. In this article, we prove that a ground state solution exists in the subcritical case if $\kappa$ is large enough and $h$ satisfies (1.3). Further, if $\kappa$ becomes very small in this case then there does not exist any solution to our system. The study in the critical case, i.e. $s_1=s_2=s, \alpha+\beta=2_s$, is more complex and the solution exists only for large $\kappa$ and radial $h$ satisfying (H1). Finally, we establish a Pohozaev identity which enables us to prove the non-existence results under some smooth assumptions on $h$.

math.AP↗
arXiv2023

Normalized Ground State Solutions for Fractional Schrodinger Systems with General Nonlinearities

Normalized ground state solutions (NGSS) of Schrodinger equations (SE) have attracted the attention of many research groups during the last decades. This is essentially due to their relevance in many fields in physics and engineering, where the stable and most attractive solutions happen to be the normalized ones. For a single (SE), recent developments lead to the establishment of existence and non-existence results for a wide range of natural nonlinearities in (SE) in the sub-critical, critical and super-critical regimes. However for systems of (SE), there are still many interesting open questions for basic nonlinearities. It certainly requires innovative ideas to shed some light to treat these complex situations. So far, only a very few specific nonlinearities have been addressed. Unlike the single (SE), the corresponding strict sub-additivity inequality is challenging and an improved concentration-compactness theorem is critical to treat (NGSS) of systems of (SE). The aim of this paper is to establish the existence of (NGSS) for a large class of nonlinearities. This class includes many relevant pure-power type nonlinearities and can be easily extended to Hartree type nonlinearities (and a combination of both). The presence of the fractional Laplacian adds a considerable difficulty to rule out the dichotomy. We were able to overcome this challenge and establish very general assumptions ensuring the strict subadditivity of the constrained energy functional. We believe that our approach will open the door to many unresolved problems.

math.AP↗
arXiv2023

Two Positive Normalized Solutions and Phase Separation for Coupled Schr\"odinger Equations on Bounded Domain with L2-Supercritical and Sobolev Critical or Subcritical Exponent

In this paper we study the existence of positive normalized solutions of the following coupled Schr\"{o}dinger system: \begin{align} \left\{ \begin{aligned} & -\Delta u = \lambda_u u + \mu_1 u^3 + \beta uv^2, \quad x \in \Omega, \\ & -\Delta v = \lambda_v v + \mu_2 v^3 + \beta u^2 v, \quad x \in \Omega, \\ & u > 0, v > 0 \quad \text{in } \Omega, \quad u = v = 0 \quad \text{on } \partial\Omega, \end{aligned} \right. \nonumber \end{align} with the $L^2$ constraint \begin{align} \int_{\Omega}|u|^2dx = c_1, \quad \quad \int_{\Omega}|v|^2dx = c_2, \nonumber \end{align} where $\mu_1, \mu_2 > 0$, $\beta \neq 0$, $c_1, c_2 > 0$, and $\Omega \subset \mathbb{R}^N$ ($N = 3, 4$) is smooth, bounded, and star-shaped. Note that the nonlinearities and the coupling terms are both $L^2$-supercritical in dimensions 3 and 4, Sobolev subcritical in dimension 3, Sobolev critical in dimension 4. We show that this system has a positive normalized solution which is a local minimizer. We further show that the system has a second positive normalized solution, which is of M-P type when $N = 3$. This seems to be the first existence result of two positive normalized solutions for such a Schr\"{o}dinger system, especially in the Sobolev critical case. We also study the limit behavior of the positive normalized solutions in the repulsive case $\beta \to -\infty$, and phase separation is expected.

math.AP↗
arXiv2023

On the Number of Normalized Ground State Solutions for a class of Elliptic Equations with general nonlinearities and potentials

We provide a precise description of the set of normalized ground state solutions (NGSS) for the class of elliptic equations: $$ -\Delta u - \lambda u + V (| x |) u - f (| x |, u) = 0,\quad\text{in}\quad \mathbb{R}^n,\ n\geq 1. $$ In particular, we show that under suitable assumptions on $V$ and $f$, the NGSS is unique for all the masses except for at most a finite number. Moreover, we prove that when unique, the NGSS $u_c$ is a smooth function of the mass $c.$ Our method is as follow: using the NGSS for a given mass $c$, we construct an exhaustive list of potential candidates to the minimization problem for masses close to $c$, and we develop a strategy how to pick the right one. In particular, if there is a unique NGSS for a given mass $c_0,$ then this uniqueness property is inherited for all the masses $c$ close to $c_0.$ Our method is general and applies to other equations provided that some key properties hold true.

math.AP↗
arXiv2023

On the focusing fractional nonlinear Schr\"{o}dinger equation on the waveguide manifolds

In this paper, we consider the focusing fractional nonlinear Schr\"{o}dinger equation (FNLS) on the waveguide manifolds $\mathbb{R}^d\times\mathbb{T}^m$ both in the isotropic and anisotropic case. Under different conditions, we establish the existence and periodic dependence of the ground states of the focusing FNLS. In the intercritical regime, we also establish the large data scattering for the anisotropic focusing FNLS by appealing to the framework of semivirial vanishing geometry.

math.AP↗
arXiv2023

On radial positive normalized solutions of the Nonlinear Schr\"odinger equation in an annulus

We are interested in the following semilinear elliptic problem: \begin{equation*} \begin{cases} -\Delta u + \lambda u = u^{p-1} \ \text{in} \ T,\\ u > 0, u = 0 \ \text{on} \ \partial T,\\ \int_{T}u^{2} \, dx= c \end{cases} \end{equation*} where $T = \{x \in \mathbb{R}^{N}: 1 < |x| < 2\}$ is an annulus in $\mathbb{R}^{N}$, $N \geq 2$, $p > 1$ is Sobolev-subcritical, searching for conditions (about $c$, $N$ and $p$) for the existence of positive radial solutions. We analyze the asymptotic behavior of $c$ as $\lambda \rightarrow +\infty$ and $\lambda \rightarrow -\lambda_1$ to get the existence, non-existence and multiplicity of normalized solutions. Additionally, based on the properties of these solutions, we extend the results obtained in \cite{pierotti2017normalized}. In contrast of the earlier results, a positive radial solution with arbitrarily large mass can be obtained when $N \geq 3$ or if $N = 2$ and $p < 6$. Our paper also includes the demonstration of orbital stability/instability results.

math.AP↗
arXiv2023

Existence and instability of standing waves for the biharmonic nonlinear Schroedinger equation with combined nonlinearities

We prove the existence of normalized ground state solutions for the biharmonic Schr\"odinger equation with combined nonlinearities and show that all ground states correspond to the local minima of the associated energy functional restricted to the appropriate set. Moreover, we prove that the standing waves are strongly unstable by blowup. In particular, our results cover the critical case. Our method is novel and innovative as previous ideas cannot be used in many cases under this study.

math.AP↗
arXiv2023

Positive solution for an elliptic system with critical exponent and logarithmic terms

In this paper, we study the existence and nonexistence of positive solutions for a coupled elliptic system with critical exponent and logarithmic terms. The presence of the the logarithmic terms brings major challenges and makes it difficult to use the previous results established in the work of Chen and Zou without new ideas and innovative techniques.

math.AP↗
arXiv2023

On the Eigenvalues of the $p\&q-$ Fractional Laplacian

We consider the eigenvalue problem for the fractional $p \& q-$Laplacian \begin{equation} \left\{\begin{aligned} (- \Delta)_p^{s}\, u + \mu(- \Delta)_q^{s}\, u+ |u|^{p-2}u+\mu|u|^{q-2}u=\lambda\ V(x)|u|^{p-2}u\quad & \text{in } \Omega\\ u=0\quad& \text{in}\quad\R^N\backslash\Omega, \end{aligned}\right. \end{equation} where $\Omega$ is an open bounded, and possibly disconnected domain, $\lambda\in\R$, $1 0$ with a weight function in $L^\infty(\Omega)$ that is allowed no change sign. We show that the problem has a continuous spectrum. Moreover, our result reveals a discontinuity property for the spectrum as the parameter $\mu\to 0^+.$ In addition, a stability property of eigenvalues as $s\to 1^-$ is established.

math.AP↗
arXiv2023

Uniqueness of normalized ground states for NLS models

We present two methods to prove the uniqueness of normalized ground states. We will first discuss the key ideas and ingredients of each method. Then, we will apply them to various classes of PDEs. Our approach is applicable to other operators, domains and nonlinearities provided that some hypotheses are satisfied.

math.AP↗