Analysis of PDEs (math.AP)

Abstract: We consider weak solutions to very singular parabolic equations involving both one-Laplace-type operators, which have anisotropic diffusivity, and $p$-Laplace-type operators with $\frac{2n}{n+2}<p<\infty$, where $n\ge 2$ denotes the space dimension. This equation becomes no longer uniformly parabolic near a facet, the place where a spatial gradient vanishes. The aim of this paper is to prove that spatial derivatives of weak solutions are continuous even across facets. This is possible by showing local a priori H\"{o}lder continuity of gradients suitably truncated near facets. To give a rigorous proof, we consider an approximating parabolic problems, and appeal to standard methods including De Giorgi's truncation and comparisons to Dirichlet heat flows.

Abstract: We study the well-posedness of the Cauchy problem for the Faraday tensor on globally hyperbolic manifolds with timelike boundary. The existence of Green operators for the operator $\mathrm{d}+\delta$ and a suitable pre-symplectic structure on the space of solutions are discussed.

Abstract: In this article, the Tricomi problem for a parabolic-hyperbolic type equation in a mixed domain is investigated. Riemann-Liouville fractional derivative participates in the parabolic part of the considerated equation, and the hyperbolic part consists of a degenerate hyperbolic equation of the second kind.

Abstract: We prove the local H\"older regularity of the weak solution of the mixed local nonlocal parabolic equation of the form \begin{equation*} u_t-\Delta u+\text{P.V}\int_{\mathbb{R}^{n}} {\frac{u(x,t)-u(y,t)}{{\left|x-y\right|}^{n+2s}}}dy=0, \end{equation*} for $0<s<1$, for all exponent $\alpha_0\in(0,1)$. Next we show that the gradient of the weak solution is also H\"older continuous for some $\alpha\in (0,1)$. Our approach is purely analytic and it is based on perturbation techniques.

Abstract: In this paper, we mainly focus on the rigorous convergence analysis for two fully decoupled, unconditional energy stable methods of the two-phase magnetohydrodynamics (MHD) model, which described in our previous work \cite{2022Highly}. The two methods consist of semi-implicit stabilization method/invariant energy quadratization (IEQ) method \cite{2019EfficientCHEN, Yang2016Linear, Yang2017Efficient, 2019EfficientYANG} for the phase field system, the pressure projection correction method for the saddle point MHD system, the exquisite implicit-explicit treatments for nonlinear coupled terms, which leads to only require solving a sequence of small elliptic equations at each time step. As far as we know, it's the first time to establish the optimal convergence analysis of fully decoupled and unconditional energy stable methods for multi-physics nonlinear two-phase MHD model. In addition, several numerical examples are showed to test the accuracy and stability of the presented methods.

Abstract: In this paper we devise a deep learning algorithm to find non-trivial zeros of Fokker-Planck operators when the drift is non-solenoidal. We demonstrate the efficacy of our algorithm for problem dimensions ranging from 2 to 10. Our method scales linearly with dimension in memory usage. We show that this method produces better approximations compared to Monte Carlo methods, for the same overall sample sizes, even in low dimensions. Unlike the Monte Carlo methods, our method gives a functional form of the solution. We also demonstrate that the associated loss function is strongly correlated with the distance from the true solution, thus providing a strong numerical justification for the algorithm. Moreover, this relation seems to be linear asymptotically for small values of the loss function.

Abstract: This paper is devoted to the lifespan estimates of small classical solutions of the initial value problems for one dimensional wave equations with semilinear terms of the spatial derivative of the unknown function. It is natural that the result is same as the one for semilinear terms of the time-derivative. But there are so many differences among their proofs. Moreover, it is meaningful to study this problem in the sense that it may help us to investigate its blow-up boundary in the near future.

Abstract: In this paper, we obtain upper bounds for the critical time $T^*$ of the blow-up for the parabolic-elliptic Patlak-Keller-Segel system on the 2D-Euclidean space. No moment condition or/and entropy condition are required on the initial data; only the usual assumptions of non-negativity and finiteness of the mass is assumed. The result is expressed not only in terms of the supercritical mass $M> 8\pi$, but also in terms of the {\it shape} of the initial data.

Abstract: In this work we prove a Strichartz estimate for the Schr\"odinger equation in the quasiperiodic setting. We also show a lower bound on the number of resonant frequency interactions in this situation.

Abstract: We consider the question of existence of weak solutions for the fully inhomogeneous, stationary generalized Navier-Stokes equations for homogeneous, shear-thinning fluids. For a shear rate exponent $p \in \big(\tfrac{2d}{d+1}, 2\big)$, previous results require either smallness of the norm or vanishing of the normal component of the boundary data. In this work, combining previous methods, we propose a new, more general smallness condition.

Abstract: We give an explicit description of the spectral closure for the three-dimensional linear Boltzmann-BGK equation in terms of the macroscopic fields, density, flow velocity and temperature. This results in a new linear fluid dynamics model which is valid for any relaxation time. The non-local exact fluid dynamics equations are compared to the Euler, Navier--Stokes and Burnett equations. Our results are based on a detailed spectral analysis of the linearized Boltzmann-BGK operator together with a suitable choice of spectral projection.

Abstract: A kind of nonlocal reaction-diffusion equations on an unbounded domain containing fractional Laplacian operator is analyzed. To be precise, we prove the convergence of solutions of the equation governed by the fractional Laplacian to the solutions of the classical equation governed by the standard Laplacian, when the fractional parameter grows to 1. The existence of global attractors is investigated as well. The novelty of this paper is concerned with the convergence of solutions when the fractional parameter varies, which, as far as the authors are aware, seems to be the first result of this kind of problems in the literature.

Abstract: We study a generalized class of supersolutions, so-called supercaloric functions to the porous medium equation in the fast diffusion case. Supercaloric functions are defined as lower semicontinuous functions obeying a parabolic comparison principle. We prove that bounded supercaloric functions are weak supersolutions. In the supercritical range, we show that unbounded supercaloric functions can be divided into two mutually exclusive classes dictated by the Barenblatt solution and the infinite point-source solution, and give several characterizations for these classes. Furthermore, we study the pointwise behavior of supercaloric functions and obtain connections between supercaloric functions and weak supersolutions.

Abstract: We show that two different notions of solutions to the obstacle problem for the porous medium equation, a potential theoretic notion and a notion based on a variational inequality, coincide for regular enough compactly supported obstacles.