72 research outputs found
Generalized Finite Element Systems for smooth differential forms and Stokes problem
We provide both a general framework for discretizing de Rham sequences of
differential forms of high regularity, and some examples of finite element
spaces that fit in the framework. The general framework is an extension of the
previously introduced notion of Finite Element Systems, and the examples
include conforming mixed finite elements for Stokes' equation. In dimension 2
we detail four low order finite element complexes and one infinite family of
highorder finite element complexes. In dimension 3 we define one low order
complex, which may be branched into Whitney forms at a chosen index. Stokes
pairs with continuous or discontinuous pressure are provided in arbitrary
dimension. The finite element spaces all consist of composite polynomials. The
framework guarantees some nice properties of the spaces, in particular the
existence of commuting interpolators. It also shows that some of the examples
are minimal spaces.Comment: v1: 27 pages. v2: 34 pages. Numerous details added. v3: 44 pages. 8
figures and several comments adde
Constructions of some minimal finite element systems
Within the framework of finite element systems, we show how spaces of
differential forms may be constructed, in such a way that they are equipped
with commuting interpolators and contain prescribed functions, and are minimal
under these constraints. We show how various known mixed finite element spaces
fulfill such a design principle, including trimmed polynomial differential
forms, serendipity elements and TNT elements. We also comment on virtual
element methods and provide a dimension formula for minimal compatible finite
element systems containing polynomials of a given degree on hypercubes.Comment: Various minor changes, based on suggestions of paper referee
On variational eigenvalue approximation of semidefinite operators
Eigenvalue problems for semidefinite operators with infinite dimensional
kernels appear for instance in electromagnetics. Variational discretizations
with edge elements have long been analyzed in terms of a discrete compactness
property. As an alternative, we show here how the abstract theory can be
developed in terms of a geometric property called the vanishing gap condition.
This condition is shown to be equivalent to eigenvalue convergence and
intermediate between two different discrete variants of Friedrichs estimates.
Next we turn to a more practical means of checking these properties. We
introduce a notion of compatible operator and show how the previous conditions
are equivalent to the existence of such operators with various convergence
properties. In particular the vanishing gap condition is shown to be equivalent
to the existence of compatible operators satisfying an Aubin-Nitsche estimate.
Finally we give examples demonstrating that the implications not shown to be
equivalences, indeed are not.Comment: 26 page
Foundations of finite element methods for wave equations of Maxwell type
The first part of the paper is an overview of the theory of approximation of wave equations by Galerkin methods. It treats convergence theory for linear second order evolution equations and includes studies of consistency and eigenvalue approximation.
We emphasize differential operators, such as the curl, which have large kernels and use L2 stable interpolators preserving them.
The second part is devoted to a framework for the construction of finite element spaces of differential forms on cellular complexes.
Material on homological and tensor algebra as well as differential and discrete geometry is included. Whitney forms, their duals, their high order versions, their tensor products and their hp-versions all fit
Upwinding in finite element systems of differential forms
We provide a notion of finite element system, that enables the construction spaces of differential forms, which can be used for the numerical solution of variationally posed partial difeerential equations. Within this framework, we introduce a form of upwinding, with the aim of stabilizing methods for the purposes of computational uid dynamics, in the vanishing viscosity regime.
Published as the Smale Prize Lecture in: Foundations of computational mathematics, Budapest 2011, London Mathematical Society Lecture Note Series, 403, Cambridge University Press, 2013
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