非线性抛物方程混合有限元方法的高精度分析

采用双线性元及零阶Raviart-Thomas元（Q₁₁+Q₁₀×Q₀₁）对非线性抛物方程讨论了一种H¹-Galerkin混合有限元方法.提出一个线性化的二阶格式,利用数学归纳法有技巧的导出了原始变量u在H¹（Ω）模意义下及流量p=▽u在L²（Ω）模意义下的O（h²+τ²）阶超逼近性质.引入一个有关初始点的时间离散方程,并利用其得到了▽ ·在L²（Ω）模意义下的O（h²+τ²）阶的超逼近结果.同时利用插值后处理技巧得到整体超收敛.最后,数值算例结果验证了理论分析（其中,h是剖分参数,τ是时间步长）.     

抛物方程的一个新混合元格式的高精度分析

借助双二次元及一阶Raviart-Thomas(R-T)元对抛物方程提出了一种新的协调混合有限元格式,导出了半离散及全离散格式下原始变量在H¹和L1和L²模意义下以及流量(?)在L2模意义下以及流量(?)在L²模意义下的超逼近结果.     

非线性湿气迁移方程的超收敛分析

本文用混合有限元方法研究一般的非线性湿气迁移方程.利用双线性元Q₁₁和零阶Raviart-Thomas元(Q₁₀×Q₀₁)证明方程的超收敛性.利用这两个单元插值算子的性质和平均值技巧,得到了方程半离散格式的O(h²)阶超收敛结果.对于方程线性化的Crank-Nicolson(C-N)全离散格式,得到了具有O(h²+τ²)阶的超收敛结果,这里h是空间剖分参数,τ是时间步长.该方法说明如果线性化问题有超收敛性,那么对应的非线性问题有同样的超收敛性.最后,给出数值算例,证实了理论分析的正确性和方法的有效性.     

强阻尼波动方程的H1-Galerkin混合有限元超收敛分析

研究了强阻尼波动方程的H¹-Galerkin混合有限元方法的超收敛性. 借助于协调线性三角形元已有的分析估计式, 直接利用插值算子代替原始变量 u 的 Ritz 投影和应力变量 p 的 Ritz-Volterra 投影,对半离散和全离散格式, 得到了u在 H¹(Ω) 模和 p 在 H(div;Ω) 模意义下比以往文献高一阶的超逼近和超收敛结果.     

抛物型方程一个新的非协调混合元超收敛性分析及外推

本文研究了抛物型方程在新混合元格式下的非协调混合有限元方法. 在抛弃传统有限元分析的必要工具-Ritz 投影算子的前提下,直接利用单元的插值性质,运用高精度分析和对时间t的导数转移技巧,借助于插值后处理技术,分别导出了关于原始变量u的H¹-模和通量p=▽u在L²-模下的O（h²）阶超逼近性质和整体超收敛. 进一步,通过构造合适的辅助问题,运用Richardson 外推格式,得到了具有更高精度O（h³）阶的外推结果. 最后,给出了一些数值结果验证了理论分析的正确性.     

Sine-Gordon方程的H~1-Galerkin非协调混合有限元分析

研究一类Sine-Gordon方程的H¹-Galerkin非协调混合有限元方法,在矩形网格剖分下,在不需要满足LBB相容性条件及不采用传统的Ritz投影的情况下,得到了与协调有限元方法相同的L1-Galerkin非协调混合有限元方法,在矩形网格剖分下,在不需要满足LBB相容性条件及不采用传统的Ritz投影的情况下,得到了与协调有限元方法相同的L²模和H2模和H¹模的误差估计,进一步拓展了H1模的误差估计,进一步拓展了H¹-Galerkin混合有限元的应用范围.     

非线性色散耗散波动方程一个新的低阶H1-Galerkin混合有限元方法(英文)

本文利用最简单的双线性矩形元和零阶Raviart-Thomas(简写为R-T)元研究了一类非线性的色散耗散波动方程的低阶H¹-Galerkin混合有限元方法(简写为FEM).利用插值算子代替传统的Ritz投影,再结合积分恒等式技巧,导出了半离散格式和全离散格式下,u的H¹模和■的H(div,?)模的超逼近结果,从而改进已有文献的结果.最后,数值结果验证了理论分析的有效性.     

Schrödinger方程的连续时空有限元方法

本文讨论Schr?dinger方程的连续时空有限元方法,通过引入相应的时空投影算子,利用实部虚部分离技巧,得到了变量u在时间节点处的L²范数,以及u和u_t的全局L²(H¹)和L²(L²)范数意义下的最优误差估计结果.该文的结论对进一步探索和设计Schr?dinger方程的数值算法是有益的.     

各向异性EQ1rot非协调元高精度分析的一般格式

本文在各向异性网格下讨论了一般二阶椭圆方程的EQ₁^rot非协调有限元逼近. 利用Taylor展开, 积分恒等式和平均值技巧导出了一些关于该元新的高精度估计. 再结合该元所具有的二个特殊性质: (a)当精确解属于H³时, 其相容误差为O(h²)阶比它的插值误差高一阶; (b)插值算子与Ritz投影算子等价,得到了在能量模意义下O(h²)阶的超逼近性质. 进而,借助于插值后处理技术给出了整体超收敛的一般估计式.     

对流占优问题稳定化的扩展混合有限元方法

讨论了对流占优问题稳定化的扩展混合元数值模拟.把稳定化的思想与扩展混合元方法相结合,既可以高精度逼近未知函数,未知函数的梯度及伴随向量函数,又能保证格式的稳定性.理论分析表明,方法是有效的,具有最优L²逼近精度.     

对流扩散反应方程的局部投影稳定化连续时空有限元方法

本文将局部投影稳定化(LPS)方法和连续时空有限元方法相结合研究对流扩散反应方程,给出稳定化连续时空有限元离散格式.与传统的时空有限元研究思路不同,时间方向利用Lagrange插值多项式,解耦时间和空间变量,降低时空有限元解的维数,具有减少计算量和简化理论分析的优点.通过引入Legendre多项式给出了有限元解的稳定性...     

Asymptotic and Partial Asymptotic Hankel Operators on $$H^2(\mathbb{D}^n)$$ H 2 ( D n ) )

In this paper, we generalize the concept of asymptotic Hankel operators on H²(D) to the Hardy space H²(Dⁿ) (over polydisk) in terms of asymptotic Hankel and partial asymptotic Hankel operators and investigate some properties in case of its weak and strong convergence. Meanwhile, we introduce i^th-partial Hankel operators on H²(Dⁿ) and obtain a characterization of its compactness for n > 1. Our main results include the containment of Toeplitz algebra in the collection of all strong partial asymptotic Hankel operators on H²(Dⁿ). It is also shown that a Toeplitz operator with symbol φ is asymptotic Hankel if and only if φ is holomorphic function in L^∞(Tⁿ).     

Least-squares Galerkin procedures for parabolic integro-differential equations

Two least-squares Galerkin finite element schemes are formulated to solve parabolic integro-differential equations. The advantage of this method is that it is not subject to the LBB condition. The convergence analysis shows that the least-squares mixed element schemes yield the approximate solution with optimal accuracy in H(div;Ω)×H¹(Ω) and (L²(Ω))²×L²(Ω), respectively.     

Crank–Nicolson least-squares Galerkin procedures for parabolic integro-differential equations

Two Crank–Nicolson least-squares Galerkin finite element schemes are formulated to solve parabolic integro-differential equations. The advantage of this method is that it is not subject to the LBB condition. The convergence analysis shows that the methods yield the approximate solutions with optimal accuracy in H(div; Ω) × H¹(Ω) and (L²(Ω))² × L²(Ω), respectively. Moreover, the two methods both get the approximate solutions with second-order accuracy in time increment.     

An extended result of Kleitman and Saks concerning binary trees

In a recent paper, D.J. Kleitman and M.E. Saks gave a proof of Huang's conjecture on alphabetic binary trees.

Given a set E = {e_i}, I = 0, 1, 2, …, m and assigned positive weights to its elements and supposing the elements are indexed such that w(e₀) ≤ w(e₁) ≤ … ≤w (e_m), where w(e_i) is the weight of e_i, we call the following sequence E^* a ‘saw-tooth’ sequence

E^*=(e₀,e_m,e₁,…,e_j,e_m−j,…).

Huang's conjecture is: E^* is the most expensive sequence for alphabetic binary trees. This paper shows that this property is true for the L-restricted alphabetic binary trees, where L is the maximum length of the leaves and log₂(m + 1) ≤L≤m.     

Asymptotic bounds for some bipartite graph: complete graph Ramsey numbers

The Ramsey number r(H,K_n) is the smallest integer N so that each graph on N vertices that fails to contain H as a subgraph has independence number at least n. It is shown that r(K_2,m,K_n)(m−1+o(1))(n/log n)² and r(C_2m,K_n)c(n/log n)^m/(m−1) for m fixed and n→∞. Also r(K_2,n,K_n)=Θ(n³/log² n) and .     

Non blow-up criterion for the 3-D Magneto-hydrodynamics equations in the limiting case

In this paper, we prove that suitable weak solution(u, b) of the 3-D MHD equations can be extended beyond T if u∈L~∞(0,T; L~3(R~3)) and the horizontal components b_h of the magnetic field satisfies the well-known Ladyzhenskaya-Prodi-Serrin condition, which improves the corresponding regularity criterion by Mahalov-Nicolaenko-Shilkin.     

Three-dimensional stable nonconforming parallelepiped elements for the Stokes problem

Stability result is obtained for the approximation of the stationary Stokes problem with nonconforming elements proposed by Jim Douglas Jr. et al. [Math. Model. Numer. Anal. 33 (4) (1999) 747] for the velocity with conforming bubble functions and discontinuous piecewise linear for the pressure on parallelepiped elements. Optimal order H¹ and L² error estimates are derived.     

A C finite element method for an inverse problem

A C⁰ finite element method is presented for an inverse problem in which the coefficient in the differential operator is to be determined from the measurement of the solution of a boundary value problem. The unknown in the inverse problem is approximated by a minimizer of a cost function that includes both the output error and equation error. Error estimates in a weighted H⁻¹ norm and L² are given. Numerical examples are presented to show features of the method.