Analysis of PDEs (math.AP)

Abstract: In an inviscid and incompressible fluid in dimension 3, we prove the existence of several helical filaments, or vortex helices, collapsing into each others.

Abstract: This work relates quantitatively homogenization to Anderson localization for heterogeneous acoustic operators: we draw consequences on the spatial spreading of eigenstates in the lower spectrum (if any) from the long-time homogenization of the wave equation, through dispersive estimates. This gives an alternative proof (avoiding Floquet theory) that the lower spectrum of the acoustic operator is purely absolutely continuous in case of periodic coefficients, and it further provides nontrivial quantitative lower bounds on the spatial spreading of potential eigenstates in case of quasiperiodic and random coefficients.

Abstract: These notes present Sobolev-Gagliardo-Nirenberg endpoint estimates for classes of homogeneous vector differential operators. Away of the endpoint cases, the classical Calder\'on-Zygmund estimates show that the ellipticity is necessary and sufficient to control all the derivatives of the vector field. In the endpoint case, Ornstein showed that there is no nontrivial estimate on same-order derivatives. On the other hand endpoint Sobolev estimates were proved for the deformation operator (Korn-Sobolev inequality by M.J. Strauss) and for the Hodge complex (Bourgain and Brezis). The class of operators for which such Sobolev estimates holds can be characterized by a cancelling condition. The estimates rely on a duality estimate for $L^1$ vector fields satisfying some conditions on the derivatives, combined with classical algebraic and harmonic analysis techniques. This characterization unifies classes of known inequalities and extends to the case of Hardy inequalities.

Abstract: In this paper, we study the existence, uniqueness and asymptotic behaviour of almost periodic (AP-) and asymptotically almost periodic (AAP-) mild solutions to the incompressible Navier-Stokes equations on non-compact manifolds with negative Ricci curvature tensors. We use certain exponential estimates of the Stokes semigroup to prove the Massera-type principle which guarantees the wellposedness of AP and AAP- mild solutions for the inhomogeneous Stokes equations. Then, by using fixed point arguments and Gronwall's inequality we establish the wellposedness and exponential decay for global-in-time of such solutions of Navier-Stokes equations.

Abstract: We consider an energy model for harmonic graphs with junctions and study the regularity properties of minimizers and their free boundaries.

Abstract: We discuss existence results for a quasi-linear elliptic equation of critical Sobolev growth [H. Brezis, L. Nirenberg, Positive solutions of nonlinear elliptic equations involving critical Sobolev exponents, Comm. Pure Appl. Math. 36 (1983), 437--477; M. Guedda, L. Veron, Quasilinear elliptic equations involving critical Sobolev exponents, Nonlinear Anal. 13 (1989), no. 8, 879--902] in the low-dimensional case, where the problem has a global character which is encoded in sign properties of the ``regular" part for the corresponding Green's function as in [O. Druet, Elliptic equations with critical Sobolev exponents in dimension 3, Ann. Inst. H. Poincar\'e Anal. Non Lin\'eaire 19 (2002), no. 2, 125--142; P. Esposito, On some conjectures proposed by Ha\"im Brezis, Nonlinear Anal. 54 (2004), no. 5, 751--759].

Abstract: In integral geometry generalized with Aleksandrov's variational theory, Lutwak-Xi-Yang-Zhang\cite{LXYZ} recently opened the door to researching the cone-chord measures and its log-Minkowski problem stemming from the chord integrals. In this paper, we study the regularity of the chord log-Minkowski problem by using a nonlocal Gauss curvature flow equation, which aims to solve the existence result of even and smooth solutions to the chord log-Minkowski problem. Our results may be served as a bridge facilitating the relation among integral geometry, differential geometry and partial differential equations.

Abstract: In this work, we study the local wellposedness of the solution to a nonlinear elliptic-dispersive coupled system which serves as a model for a Micro-Electro-Mechanical System (MEMS). A simple electrostatically actuated MEMS capacitor device consists of two parallel plates separated by a gas-filled thin gap. The nonlinear elliptic-dispersive coupled system modelling the device combines a linear elliptic equation for the gas pressure with a semilinear dispersive equation for the gap width. We show the local-in-time existence of strict solutions for the system, by combining elliptic regularity results for the elliptic equation, Lipschitz continuous dependence of its solution on that of the dispersive equation, and then local-in-time existence for a resulting abstract dispersive problem. Semigroup approaches are key to solve the abstract dispersive problem.

Abstract: We show that the prescribed Gaussian curvature equation in $\mathbb{R}^2$ $$-\Delta u= (1-|x|^p) e^{2u},$$ has solutions with prescribed total curvature equal to $\Lambda:=\int_{\mathbb{R}^2}(1-|x|^p)e^{2u}dx\in \mathbb{R}$, if and only if $$p\in(0,2) \qquad \text{and} \qquad (2+p)\pi\le\Lambda<4\pi$$ and prove that such solutions remain compact as $\Lambda\to\bar{\Lambda}\in[(2+p)\pi,4\pi)$, while they produce a spherical blow-up as $\Lambda\uparrow4\pi$.

Abstract: We consider a double power nonlinear Schr\"odinger equation which possesses the algebraically decaying stationary solution $\phi_0$ as well as exponentially decaying standing waves $e^{i\omega t}\phi_\omega(x)$ with $\omega>0$. It is well-known from the general theory that stability properties of standing waves are determined by the derivative of $\omega\mapsto M(\omega):=\frac{1}{2}\|\phi_\omega\|_{L^2}^2$; namely $e^{i\omega t}\phi_\omega$ with $\omega>0$ is stable if $M'(\omega)>0$ and unstable if $M'(\omega)<0$. However, the stability/instability of stationary solutions is outside the general theory from the viewpoint of spectral properties of linearized operators. In this paper we prove the instability of the stationary solution $\phi_0$ in one dimension under the condition $M'(0):=\lim_{\omega\downarrow 0}M'(\omega)\in[-\infty, 0)$. The key in the proof is the construction of the one-sided derivative of $\omega\mapsto\phi_\omega$ at $\omega=0$, which is effectively used to construct the unstable direction of $\phi_0$.