#### Title

Ultraconvergence of finite element method by Richardson extrapolation for elliptic problems with constant coefficients

#### Document Type

Article

#### Publication Title

SIAM Journal on Numerical Analysis

#### Abstract

In this article, two novel Richardson extrapolation operators Pk1 and Pk2 are proposed to investigate local 2k order ultraconvergence properties of the kth order Lagrange finite element method for the second order elliptic problem with constant coefficients. Assume that x0 is an interior mesh node of the underlying mesh which is away from the boundary for a fixed distance unchanging with further mesh refinement. We show that, for both tensor product element and simplicial ℙk element, it holds |(u - Pk1uh)(x0)| ≤ ch |lnh| k+1 and |(u - Pk2 (u))(x0)| < ch | lnh|k+1, where u the finite element approximation of u, | is defined in section 1.1, and k = 1 if k = 1 and k = 0 if k > 1. Numerical results are provided to demonstrate the theoretic findings.

#### First Page

2302

#### Last Page

2322

#### DOI

10.1137/15M1031710

#### Publication Date

1-1-2016

#### Recommended Citation

He, Wen Ming; Lin, Runchang; and Zhang, Zhimin, "Ultraconvergence of finite element method by Richardson extrapolation for elliptic problems with constant coefficients" (2016). *Mathematics & Physics Faculty Publications*. 9.

https://rio.tamiu.edu/math_physics_facpubs/9