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Adaptive Image Compression via Optimal Mesh Refinement 通过优化网格细化实现自适应图像压缩
IF 1.3 4区 数学 Q3 MATHEMATICS, APPLIED Pub Date : 2024-01-01 DOI: 10.1515/cmam-2023-0097
Michael Feischl, Hubert Hackl
The JPEG algorithm is a defacto standard for image compression. We investigate whether adaptive mesh refinement can be used to optimize the compression ratio and propose a new adaptive image compression algorithm. We prove that it produces a quasi-optimal subdivision grid for a given error norm with high probability. This subdivision can be stored with very little overhead and thus leads to an efficient compression algorithm. We demonstrate experimentally, that the new algorithm can achieve better compression ratios than standard JPEG compression with no visible loss of quality on many images. The mathematical core of this work shows that Binev’s optimal tree approximation algorithm is applicable to image compression with high probability, when we assume small additive Gaussian noise on the pixels of the image.
JPEG 算法是图像压缩的事实标准。我们研究了自适应网格细化是否可用于优化压缩比,并提出了一种新的自适应图像压缩算法。我们证明,对于给定的误差规范,它能高概率地生成准最优细分网格。这种细分网格的存储开销极小,因此是一种高效的压缩算法。我们通过实验证明,新算法可以实现比标准 JPEG 压缩更好的压缩率,而且在许多图像上没有明显的质量损失。这项工作的数学核心表明,当我们假定图像像素上的加性高斯噪声很小时,Binev 的最优树近似算法适用于高概率图像压缩。
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引用次数: 0
A Posteriori Error Estimation for the Optimal Control of Time-Periodic Eddy Current Problems 时间周期涡流问题最优控制的后验误差估计
IF 1.3 4区 数学 Q3 MATHEMATICS, APPLIED Pub Date : 2023-11-27 DOI: 10.1515/cmam-2023-0119
Monika Wolfmayr
This work presents the multiharmonic analysis and derivation of functional type a posteriori estimates of a distributed eddy current optimal control problem and its state equation in a time-periodic setting. The existence and uniqueness of the solution of a weak space-time variational formulation for the optimality system and the forward problem are proved by deriving inf-sup and sup-sup conditions. Using the inf-sup and sup-sup conditions, we derive guaranteed, sharp and fully computable bounds of the approximation error for the optimal control problem and the forward problem in the functional type a posteriori estimation framework. We present here the first computational results on the derived estimates.
本文提出了分布式涡流最优控制问题的泛函型后验估计的多谐分析和推导及其在时间周期设置下的状态方程。通过推导出最优系统和正问题的一个弱空时变分公式解的存在唯一性,证明了其存在唯一性。利用if -sup和sup-sup条件,在函数型后验估计框架下,导出了最优控制问题和正演问题的近似误差的保证、尖锐和完全可计算的界。在这里,我们给出了推导估计的第一个计算结果。
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引用次数: 1
Discontinuous Galerkin Two-Grid Method for the Transient Navier–Stokes Equations 瞬态Navier-Stokes方程的不连续Galerkin双网格法
IF 1.3 4区 数学 Q3 MATHEMATICS, APPLIED Pub Date : 2023-11-24 DOI: 10.1515/cmam-2023-0035
Kallol Ray, Deepjyoti Goswami, Saumya Bajpai
In this paper, we apply a two-grid scheme to the DG formulation of the 2D transient Navier–Stokes model. The two-grid algorithm consists of the following steps: Step 1 involves solving the nonlinear system on a coarse mesh with mesh size 𝐻, and Step 2 involves linearizing the nonlinear system by using the coarse grid solution on a fine mesh of mesh size ℎ and solving the resulting system to produce an approximate solution with desired accuracy. We establish optimal error estimates of the two-grid DG approximations for the velocity and pressure in energy and L 2 L^{2} -norms, respectively, for an appropriate choice of coarse and fine mesh parameters. We further discretize the two-grid DG model in time, using the backward Euler method, and derive the fully discrete error estimates. Finally, numerical results are presented to confirm the efficiency of the proposed scheme.
本文将两网格格式应用于二维瞬态Navier-Stokes模型的DG表达式。两网格算法包括以下步骤:步骤1涉及在网格尺寸为𝐻的粗网格上求解非线性系统,步骤2涉及通过在网格尺寸为的细网格上使用粗网格解对非线性系统进行线性化,并求解结果系统以产生所需精度的近似解。我们分别在能量范数和l2 L^{2}范数中建立了速度和压力的两网格DG近似的最佳误差估计,以适当选择粗网格和细网格参数。利用后向欧拉方法对两网格DG模型进行离散化,得到了完全离散的误差估计。最后给出了数值结果,验证了所提方案的有效性。
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引用次数: 0
A 𝐶1-𝑃7 Bell Finite Element on Triangle 一个𝐶1-𝑃7三角形上的贝尔有限元
IF 1.3 4区 数学 Q3 MATHEMATICS, APPLIED Pub Date : 2023-11-22 DOI: 10.1515/cmam-2023-0068
Xuejun Xu, Shangyou Zhang
We construct a <jats:inline-formula> <jats:alternatives> <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:msup> <m:mi>C</m:mi> <m:mn>1</m:mn> </m:msup> </m:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j_cmam-2023-0068_ineq_0001.png" /> <jats:tex-math>C^{1}</jats:tex-math> </jats:alternatives> </jats:inline-formula>-<jats:inline-formula> <jats:alternatives> <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:msub> <m:mi>P</m:mi> <m:mn>7</m:mn> </m:msub> </m:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j_cmam-2023-0068_ineq_0002.png" /> <jats:tex-math>P_{7}</jats:tex-math> </jats:alternatives> </jats:inline-formula> Bell finite element by restricting its normal derivative from a <jats:inline-formula> <jats:alternatives> <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:msub> <m:mi>P</m:mi> <m:mn>6</m:mn> </m:msub> </m:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j_cmam-2023-0068_ineq_0003.png" /> <jats:tex-math>P_{6}</jats:tex-math> </jats:alternatives> </jats:inline-formula> polynomial to a <jats:inline-formula> <jats:alternatives> <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:msub> <m:mi>P</m:mi> <m:mn>5</m:mn> </m:msub> </m:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j_cmam-2023-0068_ineq_0004.png" /> <jats:tex-math>P_{5}</jats:tex-math> </jats:alternatives> </jats:inline-formula> polynomial, and its second normal derivative from a <jats:inline-formula> <jats:alternatives> <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:msub> <m:mi>P</m:mi> <m:mn>5</m:mn> </m:msub> </m:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j_cmam-2023-0068_ineq_0004.png" /> <jats:tex-math>P_{5}</jats:tex-math> </jats:alternatives> </jats:inline-formula> polynomial to a <jats:inline-formula> <jats:alternatives> <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:msub> <m:mi>P</m:mi> <m:mn>4</m:mn> </m:msub> </m:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j_cmam-2023-0068_ineq_0006.png" /> <jats:tex-math>P_{4}</jats:tex-math> </jats:alternatives> </jats:inline-formula> polynomial, on the three edges of every triangle. On one triangle, the finite element space contains the <jats:inline-formula> <jats:alternatives> <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:msub> <m:mi>P</m:mi> <m:mn>6</m:mn> </m:msub> </m:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j_cmam-2023-0068_ineq_0003.png" /> <jats:tex-math>P_{6}</jats:tex-math> </jats:alternatives> </jats:inline-formula> polynomial space. We show the method converges at order 7 in <jats:inline-formula> <jats:alternatives> <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:msup> <m:mi>L</m:mi> <m:mn>2</m:mn> </m:msup> </m:math> <jats:inline-graphic xmlns:xlin
我们在每个三角形的三条边上,通过限制c1c ^{1} - p7p_ {7} Bell有限元的法向导数从p6p_{6}多项式到p5p_{5}多项式,以及它的二阶法向导数从p5p_{5}多项式到p4p_{4}多项式,构造了c1c ^{1} - p7p_ {7} Bell有限元。在一个三角形上,有限元空间包含p6p_{6}多项式空间。我们证明了该方法在l2 ^{2}范数下收敛于7阶。通过消除c1c ^{1} - p7p_ {7} Argyris有限元边缘上的所有自由度,新单元的整体自由度从27 ^ V 27V渐近地大幅降低到12 ^ V 12V,其中三角形网格中的顶点数。当c1 C^{1} - p6p_ {6} Argyris有限元的整体自由度为19 ^ V 19V时,新单元同样精确,但更经济。数值试验表明,新单元比现有单元精度更高,且具有更少的全局未知量。
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引用次数: 0
Optimal Pressure Recovery Using an Ultra-Weak Finite Element Method for the Pressure Poisson Equation and a Least-Squares Approach for the Gradient Equation 压力泊松方程的超弱有限元法和梯度方程的最小二乘法的最优压力恢复
IF 1.3 4区 数学 Q3 MATHEMATICS, APPLIED Pub Date : 2023-11-20 DOI: 10.1515/cmam-2021-0242
Douglas R. Q. Pacheco, Olaf Steinbach
Reconstructing the pressure from given flow velocities is a task arising in various applications, and the standard approach uses the Navier–Stokes equations to derive a Poisson problem for the pressure p. That method, however, artificially increases the regularity requirements on both solution and data. In this context, we propose and analyze two alternative techniques to determine p L 2 ( Ω ) {pin L^{2}(Omega)} . The first is an ultra-weak variational formulation applying integration by parts to shift all derivatives to the test functions. We present conforming finite element discretizations and prove optimal convergence of the resulting Galerkin–Petrov method. The second approach is a least-squares method for the original gradient equation, reformulated and solved as an artificial Stokes system. To simplify the incorporation of the given velocity within the right-hand side, we assume in the derivations that the velocity field is solenoidal. Yet this assumption is not restrictive, as we can use non-divergence-free approximations and even compressible velocities. Numerical experiments confirm the optimal a priori error estimates for both methods considered.
从给定的流速重建压力是一项在各种应用中出现的任务,标准方法使用Navier-Stokes方程来推导压力p的泊松问题。然而,这种方法人为地增加了对解和数据的规则性要求。在这种情况下,我们提出并分析了两种替代技术来确定p∈l2¹(Ω) {p In L^{2}(Omega)}。第一个是一个超弱变分公式,应用分部积分法将所有导数平移到测试函数。给出了合适的有限元离散化方法,并证明了Galerkin-Petrov方法的最佳收敛性。第二种方法是对原始梯度方程采用最小二乘法,将其重新表述并求解为人工Stokes系统。为了简化将给定的速度并入右边,我们在推导中假定速度场是螺线形的。然而,这个假设不是限制性的,因为我们可以使用非散度近似,甚至是可压缩速度。数值实验证实了两种方法的最优先验误差估计。
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引用次数: 0
Adaptive Absorbing Boundary Layer for the Nonlinear Schrödinger Equation 非线性Schrödinger方程的自适应吸收边界层
4区 数学 Q3 MATHEMATICS, APPLIED Pub Date : 2023-11-01 DOI: 10.1515/cmam-2023-0096
Hans Peter Stimming, Xin Wen, Norbert J. Mauser
Abstract We present an adaptive absorbing boundary layer technique for the nonlinear Schrödinger equation that is used in combination with the Time-splitting Fourier spectral method (TSSP) as the discretization for the NLS equations. We propose a new complex absorbing potential (CAP) function based on high order polynomials, with the major improvement that an explicit formula for the coefficients in the potential function is employed for adaptive parameter selection. This formula is obtained by an extension of the analysis in [R. Kosloff and D. Kosloff, Absorbing boundaries for wave propagation problems, J. Comput. Phys. 63 1986, 2, 363–376]. We also show that our imaginary potential function is more efficient than what is used in the literature. Numerical examples show that our ansatz is significantly better than existing approaches. We show that our approach can very accurately compute the solutions of the NLS equations in one dimension, including in the case of multi-dominant wave number solutions.
摘要提出了一种非线性Schrödinger方程的自适应吸收边界层技术,该技术与分时傅立叶谱方法(TSSP)相结合,用于NLS方程的离散化。本文提出了一种新的基于高阶多项式的复吸收势函数,主要改进是采用显式公式对势函数中的系数进行自适应参数选择。该公式由[R]中的分析推广而得。波传播问题的吸收边界,J.计算机学报。物理学报,2002,23(2):363-376。我们还证明了我们的虚势函数比文献中使用的函数更有效。数值算例表明,我们的方法明显优于现有的方法。我们表明,我们的方法可以非常准确地计算一维NLS方程的解,包括在多主导波数解的情况下。
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引用次数: 0
A Conforming Virtual Element Method for Parabolic Integro-Differential Equations 抛物型积分-微分方程的一致虚元法
4区 数学 Q3 MATHEMATICS, APPLIED Pub Date : 2023-10-11 DOI: 10.1515/cmam-2023-0061
Sangita Yadav, Meghana Suthar, Sarvesh Kumar
Abstract This article develops and analyses a conforming virtual element scheme for the spatial discretization of parabolic integro-differential equations combined with backward Euler’s scheme for temporal discretization. With the help of Ritz–Voltera and L 2 L^{2} projection operators, optimal a priori error estimates are established. Moreover, several numerical experiments are presented to confirm the computational efficiency of the proposed scheme and validate the theoretical findings.
摘要结合时间离散的后向欧拉格式,提出并分析了抛物型积分微分方程空间离散化的符合虚元格式。利用Ritz-Voltera算子和l2l ^{2}投影算子,建立了最优先验误差估计。最后,通过数值实验验证了该方法的计算效率和理论结果。
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引用次数: 0
A Time Splitting Method for the Three-Dimensional Linear Pauli Equation 三维线性泡利方程的时间分裂方法
4区 数学 Q3 MATHEMATICS, APPLIED Pub Date : 2023-10-06 DOI: 10.1515/cmam-2023-0094
Timon S. Gutleb, Norbert J. Mauser, Michele Ruggeri, Hans Peter Stimming
Abstract We analyze a numerical method to solve the time-dependent linear Pauli equation in three space dimensions. The Pauli equation is a semi-relativistic generalization of the Schrödinger equation for 2-spinors which accounts both for magnetic fields and for spin, with the latter missing in preceding numerical work on the linear magnetic Schrödinger equation. We use a four term operator splitting in time, prove stability and convergence of the method and derive error estimates as well as meshing strategies for the case of given time-independent electromagnetic potentials, thus providing a generalization of previous results for the magnetic Schrödinger equation.
摘要分析了三维空间中时变线性泡利方程的数值求解方法。泡利方程是对2旋量Schrödinger方程的半相对论推广,该方程考虑了磁场和自旋,而后者在之前对线性磁Schrödinger方程的数值工作中缺失。我们使用四项算子在时间上分裂,证明了该方法的稳定性和收敛性,并推导了给定时无关电磁势情况下的误差估计和网格划分策略,从而为磁性Schrödinger方程的先前结果提供了推广。
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引用次数: 0
Convergence of the Incremental Projection Method Using Conforming Approximations 使用一致性逼近的增量投影法的收敛性
4区 数学 Q3 MATHEMATICS, APPLIED Pub Date : 2023-10-05 DOI: 10.1515/cmam-2023-0038
Robert Eymard, David Maltese
Abstract We prove the convergence of an incremental projection numerical scheme for the time-dependent incompressible Navier–Stokes equations, without any regularity assumption on the weak solution. The velocity and the pressure are discretized in conforming spaces, whose compatibility is ensured by the existence of an interpolator for regular functions which preserves approximate divergence-free properties. Owing to a priori estimates, we get the existence and uniqueness of the discrete approximation. Compactness properties are then proved, relying on a Lions-like lemma for time translate estimates. It is then possible to show the convergence of the approximate solution to a weak solution of the problem. The construction of the interpolator is detailed in the case of the lowest degree Taylor–Hood finite element.
摘要本文证明了一类时变不可压缩Navier-Stokes方程的增量投影数值格式的收敛性,且不需要任何弱解的正则性假设。速度和压力在符合空间中离散化,其相容性由正则函数的插值器的存在性保证,该插值器保持了近似无发散性。由于先验估计,我们得到了离散近似的存在唯一性。然后证明紧性性质,依赖于时间平移估计的狮子引理。这样就有可能证明问题的弱解的近似解的收敛性。在最低次泰勒-胡德有限元的情况下,详细介绍了插值器的构造。
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引用次数: 0
Three Low Order H-Curl-Curl Finite Elements on Triangular Meshes 三角网格上的三阶h -旋度有限元
4区 数学 Q3 MATHEMATICS, APPLIED Pub Date : 2023-10-04 DOI: 10.1515/cmam-2023-0140
Shangyou Zhang
Abstract We construct three H-curl-curl finite elements. The P 2 P_{2} and P 3 P_{3} vector finite element spaces are both enriched by one common P 4 P_{4} bubble and their local degrees of freedom are 13 and 21, respectively. As there does not exist any P 1 P_{1} H-curl-curl conforming finite element, the P 1 P_{1} H-curl-curl nonconforming finite element is constructed with three additional P 4 P_{4} bubbles. Numerical tests are presented, confirming the conformity and the optimal order of convergence.
摘要构造了三个h -旋度有限元。p2p_{2}和p3p_{3}矢量有限元空间均由一个共同的p4p_{4}泡丰富,其局部自由度分别为13和21。由于不存在p1p_ {1} H-curl-curl非一致性有限元,因此在p1p_ {1} H-curl-curl非一致性有限元中附加三个p4p_{4}气泡。通过数值试验,验证了算法的一致性和最优收敛顺序。
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引用次数: 0
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Computational Methods in Applied Mathematics
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