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1. On the regularity of the solution of some nonlinear problems

   https://doi.org/10.1142/S0219199725400012  

   Chipot, Michel; Kinderlehrer, David (June 2026, Communications in Contemporary Mathematics)

   The goal of this note is to present regularity results regarding problems of the type [Formula: see text] in [Formula: see text], [Formula: see text] on [Formula: see text] which depend only on [Formula: see text] and not on the monotone function [Formula: see text]. This uses nice techniques developed by H. Brezis and collaborators. 

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2. Weighted Sobolev regularity and rate of approximation of the obstacle problem for the integral fractional Laplacian

   https://doi.org/10.1142/S021820251950057X  

   Borthagaray, Juan Pablo; Nochetto, Ricardo H.; Salgado, Abner J. (December 2019, Mathematical Models and Methods in Applied Sciences)

   We obtain regularity results in weighted Sobolev spaces for the solution of the obstacle problem for the integral fractional Laplacian [Formula: see text] in a Lipschitz bounded domain [Formula: see text] satisfying the exterior ball condition. The weight is a power of the distance to the boundary [Formula: see text] of [Formula: see text] that accounts for the singular boundary behavior of the solution for any [Formula: see text]. These bounds then serve us as a guide in the design and analysis of a finite element scheme over graded meshes for any dimension [Formula: see text], which is optimal for [Formula: see text]. 

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3. ɛ-Strong Simulation of Fractional Brownian Motion and Related Stochastic Differential Equations

   https://doi.org/10.1287/moor.2020.1078  

   Chen, Yi; Dong, Jing; Ni, Hao (May 2021, Mathematics of Operations Research)

   null (Ed.)

   Consider a fractional Brownian motion (fBM) [Formula: see text] with Hurst index [Formula: see text]. We construct a probability space supporting both B H and a fully simulatable process [Formula: see text] such that[Formula: see text] with probability one for any user-specified error bound [Formula: see text]. When [Formula: see text], we further enhance our error guarantee to the α-Hölder norm for any [Formula: see text]. This enables us to extend our algorithm to the simulation of fBM-driven stochastic differential equations [Formula: see text]. Under mild regularity conditions on the drift and diffusion coefficients of Y, we construct a probability space supporting both Y and a fully simulatable process [Formula: see text] such that[Formula: see text] with probability one. Our algorithms enjoy the tolerance-enforcement feature, under which the error bounds can be updated sequentially in an efficient way. Thus, the algorithms can be readily combined with other advanced simulation techniques to estimate the expectations of functionals of fBMs efficiently. 

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4. A virtual finite element method for two-dimensional Maxwell interface problems with a background unfitted mesh

   https://doi.org/10.1142/S0218202521500652  

   Cao, Shuhao; Chen, Long; Guo, Ruchi (December 2021, Mathematical Models and Methods in Applied Sciences)

   A virtual element method (VEM) with the first-order optimal convergence order is developed for solving two-dimensional Maxwell interface problems on a special class of polygonal meshes that are cut by the interface from a background unfitted mesh. A novel virtual space is introduced on a virtual triangulation of the polygonal mesh satisfying a maximum angle condition, which shares exactly the same degrees of freedom as the usual [Formula: see text]-conforming virtual space. This new virtual space serves as the key to prove that the optimal error bounds of the VEM are independent of high aspect ratio of the possible anisotropic polygonal mesh near the interface. 

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5. The VEM for time-harmonic Maxwell equations in inhomogeneous media with Lipschitz interface

   https://doi.org/10.1142/S0218202525500289  

   Chen, Chunyu; Guo, Ruchi; Wei, Huayi (May 2025, Mathematical Models and Methods in Applied Sciences)

   It has been extensively studied in the literature that solving Maxwell equations is very sensitive to mesh structures, space conformity and solution regularity. Roughly speaking, for almost all the methods in the literature, optimal convergence for low-regularity solutions heavily relies on conforming spaces and highly regular simplicial meshes. This can be a significant limitation for many popular methods based on broken spaces and non-conforming or polytopal meshes often used for inhomogeneous media, as the discontinuity of electromagnetic parameters can lead to quite low regularity of solutions near media interfaces. This very issue can be potentially worsened by geometric singularities, making those methods particularly challenging to apply. In this paper, we present a lowest-order virtual element method for solving an indefinite time-harmonic Maxwell equation in 2D inhomogeneous media with quite arbitrary polytopal meshes, and the media interface is allowed to have geometric singularity to cause low regularity. We employ the “virtual mesh” technique originally invented in [S. Cao, L. Chen and R. Guo, A virtual finite element method for two-dimensional Maxwell interface problems with a background unfitted mesh, Math. Models Methods Appl. Sci. 31 (2021) 2907–2936] for error analysis. This work admits three key novelties: (i) the proposed method is theoretically guaranteed to achieve robust optimal convergence for solutions with merely [Formula: see text] regularity, [Formula: see text]; (ii) the polytopal element shape can be highly anisotropic and shrinking, and an explicit formula is established to describe the relationship between the shape regularity and solution regularity; (iii) we show that the stabilization term is needed to produce optimal convergent solutions for indefinite problems. Extensive numerical experiments will be given to demonstrate the effectiveness of the proposed method. 

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