Analysis of PDEs (math.AP)

Abstract: We consider Maxwell equations on a smooth domain with perfectly conducting boundary conditions in isotropic media in two and three dimensions. In the charge-free case we recover Strichartz estimates due to Blair--Smith--Sogge for wave equations on domains up to endpoints. For the proof we suitably extend Maxwell equations over the boundary, which introduces coefficients on the full space with codimension-$1$ Lipschitz singularity. This system can be diagonalized to half-wave equations amenable to the results of Blair--Smith--Sogge. In two dimensions, we improve the local well-posedness of the Maxwell system with Kerr nonlinearity via Strichartz estimates.

Abstract: We prove global Schauder estimates for kinetic Kolmogorov equations with coefficients that are H\"older continuous in the spatial variables but only measurable in time. Compared to other available results in the literature, our estimates are optimal in the sense that the inherent H\"older spaces are the strongest possible under the given assumptions: in particular, under a parabolic H\"ormander condition, we introduce H\"older norms defined in terms of the intrinsic geometry that the operator induces on the space-time variables. The technique is based on the existence and the regularity estimates of the fundamental solution of the equation. These results are essential for studying backward Kolmogorov equations associated with kinetic-type diffusions, e.g. stochastic Langevin equation.

Abstract: This paper is concerned with the two-species chemotaxis-competition model with degenerate diffusion, \[\begin{cases} u_t = \Delta u^{m_1} - \chi_1 \nabla\cdot(u\nabla w) + \mu_1 u (1-u-a_1v), &x\in\Omega,\ t>0,\\% v_t = \Delta v^{m_2} - \chi_2 \nabla\cdot(v\nabla w) + \mu_2 v (1-a_2u-v), &x\in\Omega,\ t>0,\\% 0 = \Delta w +u+v-\overline{M}(t), &x\in\Omega,\ t>0, \end{cases}\] with $\int_\Omega w(x,t)\,dx=0$, $t>0$, where $\Omega := B_R(0) \subset \mathbb{R}^n$ $(n\ge5)$ is a ball with some $R>0$; $m_1,m_2>1$, $\chi_1,\chi_2,\mu_1,\mu_2,a_1,a_2>0$; $\overline{M}(t)$ is the spatial average of $u+v$. The purpose of this paper is to show finite-time blow-up in the sense that there is $\widetilde{T}_{\rm max}\in(0,\infty)$ such that \[\limsup_{t \nearrow \widetilde{T}_{\rm max}} (\|u(t)\|_{L^\infty(\Omega)} + \|v(t)\|_{L^\infty(\Omega)})=\infty\] for the above model within a concept of weak solutions fulfilling a moment inequality which leads to blow-up. To this end, we also give a result on finite-time blow-up in the above model with the terms $\Delta u^{m_1}$, $\Delta v^{m_2}$ replaced with the nondegenerate diffusion terms $\Delta (u+\delta)^{m_1}$, $\Delta (v+\delta)^{m_2}$, where $\delta\in(0,1]$.

Abstract: We focus on open questions regarding the uniqueness of distributional solutions of the fast diffusion equation (FDE) with a given source term. When the source is sufficiently smooth, the uniqueness follows from standard results. Assuming that the source term is a measure, the existence of different classes of solutions is known, but in many cases, their uniqueness is an open problem. In our work, we focus on the supercritical FDE and prove the uniqueness of distributional solutions with a Dirac source term that moves along a prescribed curve. Moreover, we extend a uniqueness results for the subcritical FDE from standing to moving singularities.

Abstract: In this paper, we prove the stability of viscosity solutions of the Hamilton--Jacobi equations for a sequence of networks embedded in Euclidean space. The network considered in this paper is not merely a graph -- it comprises a collection of line segments. We investigate the conditions under which the stability of viscosity solutions holds if the sequence of networks converges to some compact set in the Hausdorff sense. As a corollary, a characterization of the limit of a sequence of networks on which viscosity solutions can be considered, is obtained. In consideration of this problem, we adopt the concept of viscosity solutions as presented in the sense of Gangbo and \'{S}wi\k{e}ch.

Abstract: In this paper we prove existence of nonnegative bounded solutions for the non-autonomous prescribed mean curvature problem in non-parametric form on an open bounded domain $\Omega$ of $\mathbb{R}^N$. The mean curvature, that depends on the location of the solution $u$ itself, is asked to be of the form $f(x)h(u)$, where $f$ is a nonnegative function in $L^{N,\infty}(\Omega)$ and $h:\mathbb{R}^+\mapsto \mathbb{R}^+$ is merely continuous and possibly unbounded near zero. As a preparatory tool for our analysis we propose a purely PDE approach to the prescribed mean curvature problem not depending on the solution, i.e. $h\equiv 1$. This part, which has its own independent interest, aims to represent a modern and up-to-date account on the subject. Uniqueness is also handled in presence of a decreasing nonlinearity. The sharpness of the results is highlighted by mean of explicit examples.

Abstract: We study the existence and the properties of ground states at fixed mass for a focusing nonlinear Schr\"odinger equation in dimension two with a point interaction, an attractive Coulomb potential and a nonlinearity of power type. We prove that for any negative value of the Coulomb charge, for any positive value of the mass and for any L$^2$-subcritical power nonlinearity, such ground states exist and exhibit a logarithmic singularity where the interaction is placed. Moreover, up to multiplication by a phase factor, they are positive, radially symmetric and decreasing. An analogous result is obtained also for minimizers of the action restricted to the Nehari manifold, getting the existence also in the L$^2$-critical and supercritical cases.

Abstract: 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.