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Deng Zhang

Publications and source records attributed to Deng Zhang.

At least 37 records · Page 2Linked to original sources
arXiv2022

Sharp non-uniqueness of weak solutions to 3D magnetohydrodynamic equations

We prove the non-uniqueness of weak solutions to 3D hyper viscous and resistive MHD in the class $L^γ_tW^{s,p}_x$, where the exponents $(s,γ,p)$ lie in two supercritical regimes. The result reveals that the scaling-invariant Ladyženskaja-Prodi-Serrin (LPS) condition is the right criterion to detect non-uniqueness, even in the highly viscous and resistive regime beyond the Lions exponent. In particular, for the classical viscous and resistive MHD, the non-uniqueness is sharp near the endpoint $(0,2,\infty)$ of the LPS condition. Moreover, the constructed weak solutions admit the partial regularity outside a small fractal singular set in time with zero $\mathcal{H}^{η_*}$-Hausdorff dimension, where $η_*$ can be any given small positive constant. Furthermore, we prove the strong vanishing viscosity and resistivity result, which yields the failure of Taylor's conjecture along some subsequence of weak solutions to the hyper viscous and resistive MHD beyond the Lions exponent.

math.AP↗
arXiv2022

Sharp non-uniqueness for the 3D hyperdissipative Navier-Stokes equations: above the Lions exponent

We study the 3D hyperdissipative Navier-Stokes equations on the torus, where the viscosity exponent $α$ can be larger than the Lions exponent $5/4$. It is well-known that, due to Lions [55], for any $L^2$ divergence-free initial data, there exist unique smooth Leray-Hopf solutions when $α\geq 5/4$. We prove that even in this high dissipative regime, the uniqueness would fail in the supercritical spaces $L^γ_tW^{s,p}_x$, in view of the generalized Ladyženskaja-Prodi-Serrin condition. The non-uniqueness is proved in the strong sense and, in particular, yields the sharpness at two endpoints $(3/p+1-2α, \infty, p)$ and $(2α/γ+1-2α, γ, \infty)$. Moreover, the constructed solutions are allowed to coincide with the unique Leray-Hopf solutions near the initial time and, more delicately, admit the partial regularity outside a fractal set of singular times with zero Hausdorff $\mathcal{H}^{η_*}$ measure, where $η_*>0$ is any given small positive constant. These results also provide the sharp non-uniqueness in the supercritical Lebesgue and Besov spaces. Furthermore, the strong vanishing viscosity result is obtained for the hyperdissipative Navier-Stokes equations.

math.AP↗
arXiv2022

Non-uniqueness of weak solutions to 3D magnetohydrodynamic equations

We prove the non-uniqueness of weak solutions to 3D magnetohydrodynamic (MHD for short) equations. The constructed weak solutions do not conserve the magnetic helicity and can be close to any given smooth, divergence-free and mean-free velocity and magnetic fields. Furthermore, we prove that the weak solutions constructed by Beekie-Buckmaster-Vicol [2] for the ideal MHD can be obtained as a strong vanishing viscosity and resistivity limit of a sequence of weak solutions to MHD equations. This shows that, in contrast to the weak ideal limits, Taylor's conjecture does not hold along the vanishing viscosity and resistivity limits. Unlike in the context of the NSE [13] and the ideal MHD [2], new types of velocity and magnetic flows, featuring both the refined spatial and temporal intermittency, are constructed to respect the geometry of MHD and to control the strong viscosity and resistivity. Compatible algebraic structure is derived in the convex integration scheme. More interestingly, the new intermittent flows indeed enable us to prove the aforementioned results for the hyper-viscous and hyper-resistive MHD equations up to the sharp exponent $5/4$, which coincides exactly with the Lions exponent for 3D hyper-viscous NSE.

math.AP↗
arXiv2022

Ill-posedness of quintic fourth order Schrödinger equation

We prove that the solution map, associated to the quintic fourth order nonlinear Schrödinger equation, exhibits the norm inflation phenomenon at every point in the Sobolev spaces of super-critical regularity. Indeed, we prove this result separately in the cases of negative and of positive regularity. In the negative regularity case, we prove the result for both the defocusing and focusing equations: in the one dimensional case, the associated solution map exhibits norm inflation in Sobolev spaces of super-critical regularity (including the critical index); in the higher dimensional case, the solution map exhibits the same phenomenon in spaces of negative regularity. Meanwhile, in the case of positive regularity, we prove the result for the defocusing equation in dimensions $d=3,4,5$. Our proofs are based on the "high-to-low" and "low-to-high" frequency cascades respectively.

math.AP↗
arXiv2021

Multi solitary waves to stochastic nonlinear Schrödinger equations

In this paper, we present a pathwise construction of multi-soliton solutions for focusing stochastic nonlinear Schrödinger equations with linear multiplicative noise, in both the $L^2$-critical and subcritical cases. The constructed multi-solitons behave asymptotically as a sum of $K$ solitary waves, where $K$ is any given finite number. Moreover, the convergence rate of the remainders can be of either exponential or polynomial type, which reflects the effects of the noise in the system on the asymptotical behavior of the solutions. The major difficulty in our construction of stochastic multi-solitons is the absence of pseudo-conformal invariance. Unlike in the deterministic case [47,54], the existence of stochastic multi-solitons cannot be obtained from that of stochastic multi-bubble blow-up solutions in [54,57]. Our proof is mainly based on the rescaling approach in [39], relying on two types of Doss-Sussman transforms, and on the modulation method in [16,44], in which the crucial ingredient is the monotonicity of the Lyapunov type functional constructed by Martel, Merle and Tsai [45]. In our stochastic case, this functional depends on the Brownian paths in the noise.

math.PR↗
arXiv2021

Multi-bubble Bourgain-Wang solutions to nonlinear Schrödinger equation

We consider a general class of focusing $L^2$-critical nonlinear Schrödinger equations with lower order perturbations, for which the pseudo-conformal symmetry and the conservation law of energy are absent. In dimensions one and two, we construct Bourgain-Wang type solutions concentrating at $K$ distinct singularities, $1\leq K<\infty$, and prove that they are unique if the asymptotic behavior is within the order $(T-t)^{4+}$, for $t$ close to the blow-up time $T$. These results apply to the canonical nonlinear Schrödinger equations and, through the pseudo-conformal transform, in particular yield the existence and conditional uniqueness of non-pure multi-solitons. Furthermore, through a Doss-Sussman type transform, these results also apply to stochastic nonlinear Schrödinger equations, where the driving noise is taken in the sense of controlled rough path.

math.AP↗
arXiv2021

On uniqueness of multi-bubble blow-up solutions and multi-solitons to $L^2$-critical nonlinear Schrödinger equations

We are concerned with the focusing $L^2$-critical nonlinear Schrödinger equations in $\mathbb{R}^d$ for $d=1,2$. The uniqueness is proved for a large energy class of multi-bubble blow-up solutions, which converge to a sum of $K$ pseudo-conformal blow-up solutions particularly with low rate $(T-t)^{0+}$, as $t\to T$, $1\leq K<\infty$. Moreover, we also prove the uniqueness in the energy class of multi-solitons which converge to a sum of $K$ solitary waves with convergence rate $(1/t)^{2+}$, as $t\to \infty$. The uniqueness class is further enlarged to contain the multi-solitons with even lower convergence rate $(1/t)^{\frac 12+}$ in the pseudo-conformal space. The proof is mainly based on the pseudo-conformal invariance and the monotonicity properties of several functionals adapted to the multi-bubble case, the latter is crucial towards the upgradation of the convergence to the fast exponential decay rate.

math.AP↗
arXiv2020

On the multi-bubble blow-up solutions to rough nonlinear Schrödinger equations

We are concerned with the multi-bubble blow-up solutions to rough nonlinear Schrödinger equations in the focusing mass-critical case. In both dimensions one and two, we construct the finite time multi-bubble solutions, which concentrate at $K$ distinct points, $1\leq K<\infty$, and behave asymptotically like a sum of pseudo-conformal blow-up solutions in the pseudo-conformal space $Σ$ near the blow-up time. The upper bound of the asymptotic behavior is closely related to the flatness of noise at blow-up points. Moreover, we prove the conditional uniqueness of multi-bubble solutions in the case where the asymptotic behavior in the energy space $H^1$ is of the order $(T-t)^{3+ζ}$, $ζ>0$. These results are also obtained for nonlinear Schrödinger equations with lower order perturbations, particularly, in the absence of the classical pseudo-conformal symmetry and the conversation law of energy. The existence results are applicable to the canonical deterministic nonlinear Schrödinger equation and complement the previous work [43]. The conditional uniqueness results are new in both the stochastic and deterministic case.

math.PR↗
arXiv2020

Minimal mass blow-up solutions to rough nonlinear Schroedinger equations

We study the focusing mass-critical rough nonlinear Schroedinger equations, where the stochastic integration is taken in the sense of controlled rough path. We obtain the global well-posedness if the mass of initial data is below that of the ground state. Moreover, the existence of minimal mass blow-up solutions is also obtained in both dimensions one and two. In particular, these yield that the mass of ground state is exactly the threshold of global well-posedness and blow-up of solutions in the stochastic focusing mass-critical case. Similar results are also obtained for a class of nonlinear Schroedinger equations with lower order perturbations.

math.PR↗
arXiv2019

Optimal control of nonlinear stochastic differential equations on Hilbert spaces

We here consider optimal control problems governed by nonlinear stochastic equations on a Hilbert space H with nonconvex payoff, which is rewritten as a deterministic optimal control problem governed by a Kolmogorov equation in H. We prove the existence and first-order necessary condition of closed loop optimal controls for the above control problem. The strategy is based on solving a deterministic bilinear optimal control problem for the corresponding Kolmogorov equation on the space $L^2(H,ν)$, where $ν$ is the related infinitesimally invariant measure for the Kolmogorov operator.

math.PR↗
arXiv2019

Gaussian fluctuations and moderate deviations of eigenvalues in unitary invariant ensembles

We study the limiting behavior of the $k$-th eigenvalue $x_k$ of unitary invariant ensembles with Freud-type and uniform convex potentials. As both $k$ and $n-k$ tend to infinity, we obtain Gaussian fluctuations for $x_k$ in the bulk and soft edge cases, respectively. Multi-dimensional central limit theorems, as well as moderate deviations, are also proved. This work generalizes earlier results in the GUE and unitary invariant ensembles with monomial potentials of even degree. In particular, we obtain the precise asymptotics of corresponding Christoffel-Darboux kernels as well.

math.PR↗
arXiv2019

Optimal bilinear control of stochastic nonlinear Schrödinger equations: mass-(sub)critical case

We study optimal bilinear control problems for stochastic nonlinear Schrödinger equations in both the mass subcritical and critical case. For general initial data of the minimal L2 regularity, we prove the existence and first order Lagrange condition of an open loop control. Furthermore, we obtain uniform estimates of (backward) stochastic solutions in new spaces of type U2 and V2, adapted to evolution operators related to linear Schrödinger equations with lower order perturbations. In particular, we obtain a new temporal regularity of rescaled (backward) stochastic solutions, which is the key ingredient in the proof of tightness of approximating controls induced by Ekeland's variational principle.

math.AP↗
arXiv2018

Stochastic nonlinear Schroedinger equations in the defocusing mass and energy critical cases

We study the stochastic nonlinear Schroedinger equations with linear multiplicative noise, particularly in the defocusing mass-critical and energy-critical cases. For general initial data, we prove the global existence and uniqueness of solutions in both cases. When the quadratic variation of noise is globally bounded, we also obtain the rescaled scattering behavior of solutions in the spaces $L^2$, $H^1$ as well as the pseudo-conformal space. Moreover, the Stroock-Varadhan type theorem for the topological support of solutions are also obtained in the Strichartz and local smoothing spaces.

math.PR↗
arXiv2018

Scattering for stochastic nonlinear Schrödinger equations

We study the scattering behavior of global solutions to stochastic nonlinear Schrödinger equations with linear multiplicative noise. In the case where the quadratic variation of the noise is globally finite and the nonlinearity is defocusing,we prove that the solutions scatter at infinity in the pseudo-conformal space and in the energy space respectively, including the energy-critical case. Moreover, in the case where the noise is large, non-conservative and has infinite quadratic variation, we show that the solutions scatter at infinity with high probability for all energy-subcritical exponents.

math.PR↗
arXiv2017

Strichartz and local smoothing estimates for stochastic dispersive equations with linear multiplicative noise

We study a quite general class of stochastic dispersive equations with linear multiplicative noise, including especially the Schrödinger and Airy equations. The pathwise Strichartz and local smoothing estimates are derived here in both the conservative and nonconservative case. In particular, we obtain the P-integrability of constants in these estimates, where P is the underlying probability measure. Several applications are given to nonlinear problems, including local well-posedness of stochastic nonlinear Schrödinger equations with variable coefficients and lower order perturbations, integrability of global solutions to stochastic nonlinear Schrödinger equations with constant coefficients. As another consequence, we prove as well the large deviation principle for the small noise asymptotics.

math.PR↗
arXiv2016

Optimal bilinear control of nonlinear stochastic Schrödinger equations driven by linear multiplicative noise

Here is investigated the bilinear optimal control problem of quantum mechanical systems with final observation governed by a stochastic nonlinear Schrödinger equation perturbed by a linear multiplicative Wiener process. The existence of an open loop optimal control and first order Lagrange optimality conditions are derived, via Skorohod's representation theorem, Ekeland's variational principle and the existence for the linearized dual backward stochastic equation. Moreover, our approach in particular applies to the deterministic case.

math.PR↗
arXiv2015

The stochastic logarithmic Schrödinger equation

In this paper we prove global existence and uniqueness of solutions to the stochastic logarithmic Schrödinger equation with linear multiplicative noise. Our approach is mainly based on the rescaling approach and the method of maximal monotone operators. In addition, uniform estimates of solutions in the energy space $H^1(\mathbb{R}^d)$ and in an appropriate Orlicz space are also obtained here.

math.PR↗