1,721,000 research outputs found
Error estimates for semi-discrete dendritic growth
No full textSemi-discrete approximations to a mathematical model for 2-dimensional dendritic growth are analyzed. The model is a Stefan problem with interfacial structure. The semi-discrete problem uses a parametrization for the free boundary and finite elements in space. The main results are a priori error estimates for the temperature field and the parametrization of the free boundary. The optimality of their order is discussed. Further error estimates concern approximations to relevant geometric (e.g. curvature) and measuring quantities
Approximating Gradients with Continuous Piecewise Polynomial Functions
Full text linkMotivated by conforming finite element methods for elliptic problems of second order, we analyze the approximation of the gradient of a target function by continuous piecewise polynomial functions over a simplicial mesh. The main result is that the global best approximation error is equivalent to an appropriate sum in terms of the local best approximation errors on elements. Thus, requiring continuity does not downgrade local approximation capability and discontinuous piecewise polynomials essentially do not offer additional approximation power, even for a fixed mesh. This result implies error bounds in terms of piecewise regularity over the whole admissible smoothness range. Moreover, it allows for simple local error functionals in adaptive tree approximation of gradients
Convergent adaptive finite elements for the nonlinear Laplacian
No full textThe numerical solution of the homogeneous Dirichlet problem for the p-Laplacian is considered. We propose an adaptive algorithm with continuous piecewise affine finite elements and prove that the approximate solutions converge to the exact one. While the algorithm is a rather straight-forward generalization of those for the linear case p=2, the proof of its convergence is different. In particular, it does not rely on a strict error reduction
Efficient and Reliable A Posteriori Error Estimators for Elliptic Obstacle Problems
No full textA posteriori error estimators are derived for linear finite element approximations to elliptic obstacle problems. These estimators yield global upper and local lower bounds for the discretization error. Here discretization error means the sum of two contributions: the distance between continuous and discrete solution in the energy-norm and some quantity that is related to the distance of continuous and discrete contact set. Moreover, the local error indicators in the interior of the discrete contact set reduce to quantities that measure only data resolution
A posteriori error estimators, gradient recovery by averaging, and superconvergence
No full textFor the linear finite element solution to a linear elliptic model problem, we derive an error estimator based upon appropriate gradient recovery by local averaging. In contrast to popular variants like the ZZ estimator, our estimator contains some additional terms that ensure reliability also on coarse meshes. Moreover, the enhanced estimator is proved to be (locally) efficient and asymptotically exact whenever the recovered gradient is superconvergent. We formulate an adaptive algorithm that is directed by this estimator and illustrate its aforementioned properties, as well as their importance, in numerical tests
Explicit upper bounds for dual norms of residuals
No full textWe derive upper bounds for the dual norms of residuals that are explicit in terms of local Poincaré constants. Residuals are continuous linear functionals that are orthogonal to a finite element space and have a singular part supported on the skeleton of the underlying mesh. Functionals of this type play a key role in a posteriori error estimation. Our main tools are a discrete partition of unity and suitably weighted trace and Poincare' inequalities. The technique is illustrated for negative first order Sobolev norms and a dual norm arising in convection-reaction-diffusion problems
Locally efficient and reliable a posteriori error estimators for Dirichlet problems
No full textConsidering the Dirichlet problem for Poisson's equation in two and three dimensions, we derive a posteriori error estimators for finite element solutions with interpolated boundary values. The estimators are reliable and (locally) efficient with respect to the energy norm error, also in the case of discontinuous boundary values and load terms that are not square-integrable due to singularities at the boundary of the underlying domain. Moreover, we propose an adaptive algorithm based upon these estimators and test it also in nonsmooth cases of the aforementioned type: its convergence rate is optimal
A posteriori error estimators for regularized total variation of characteristic functions
No full textWe consider a non-uniformly elliptic double obstacle problem arising from ``convexifying'' and regularizing a minimization of a functional with total variation in the set of characteristic functions. We derive a posteriori estimators for the discretization error with linear finite elements, which are uniform in the regularization, incorporate computable and local information on the conditioning, vanish in the intersection of discrete and exact contact set, and are not affected by possible non-uniqueness. Moreover, we integrate these estimators in an adaptive algorithm and illustrate their properties by various numerical experiments
On the a posteriori error analysis for equations of prescribed mean curvature
No full textWe present two approaches to the a posteriori error analysis for prescribed mean curvature equations. The main difference between them concerns the estimation of the residual: without or with computable weights. In the second case, the weights are related to the eigenvalues of the underlying operator and thus provide local and computable information about the conditioning. We analyze the two approaches from a theoretical viewpoint. Moreover, we investigate and compare the performance of the derived indicators in
an adaptive procedure. Our theoretical and practical results show that it is advantageous to estimate the residual in a weighted way
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