Symmetry preserving numerical schemes for partial differential equations and their numerical tests

The method of equivariant moving frames is used to construct symmetry preserving finite difference schemes of partial differential equations invariant under finite-dimensional symmetry groups. Invariant numerical schemes for a heat equation with logarithmic source and the spherical Burgers' equation are obtained. Numerical tests show how invariant schemes can be more accurate than standard discretizations.     

Symmetry of extremal functions for the Caffarelli-Kohn-Nirenberg inequalities

We study the symmetry property of extremal functions to a family of weighted Sobolev inequalities due to Caffarelli-Kohn-Nirenberg. By using the moving plane method, we prove that all non-radial extremal functions are axially symmetric with respect to a line passing through the origin.

     

The choice of spectral element basis functions in domains with an axis of symmetry

New spectral element basis functions are constructed for problems possessing an axis of symmetry. In problems defined in domains with an axis of symmetry there is a potential problem of degeneracy of the system of discrete equations corresponding to nodes located on the axis of symmetry. The standard spectral element basis functions are modified so that the axial conditions are satisfied identically. The modified basis is employed only in spectral elements that are adjacent to the axis of symmetry. This modification of the spectral element method ensures that the nodes are the same in each element, which is not the case in other methods that have been proposed to tackle the problem along the axis of symmetry, and that there are no nodes along the axis of symmetry. The problems of Stokes flow past a confined cylinder and sphere are considered and the performance of the original and modified basis functions are compared.     

Spectra of Hamiltonians of molecule pseudorelativistic electrons in spaces of functions with permutational and point symmetry

We study the spectral properties of the Hamiltonian H _n of n pseudorelativistic electrons in the Coulomb field of k fixed nuclei in spaces of functions having arbitrary given types of permutational and point symmetry. For this operator, we establish the location of the essential spectrum, obtain two-sided estimates of the discrete spectrum counting function in terms of the counting functions of the discrete spectrum of some two-particle nonrelativistic operators, and find the leading term of the spectral asymptotics.     

Spaces of Maps into Classifying Spaces for Equivariant Crossed Complexes, II: The General Topological Group Case

The results of a previous paper on the equivariant homotopy theory of crossed complexes are generalised from the case of a discrete group to general topological groups. The principal new ingredient necessary for this is an analysis of homotopy coherence theory for crossed complexes, using detailed results on the appropriate Eilenberg–Zilber theory, and of its relation to simplicial homotopy coherence. Again, our results give information not just on the homotopy classification of certain equivariant maps, but also on the weak equivariant homotopy type of the corresponding equivariant function spaces.     

Numerical exploitation of symmetry in integral equations

Linear integral operators describing physical problems on symmetric domains often are equivariant, which means that they commute with certain symmetries, i.e., with a group of orthogonal transformations leaving the domain invariant. Under suitable discretizations the resulting system matrices are also equivariant. A method for exploiting this equivariance in the numerical solution of linear equations and eigenvalue problems via symmetry reduction is described. A very significant reduction in the computational expense in both the assembling of the system matrix and in solving linear systems can be obtained in this way. This reduction is particularly important because the system matrices are typically full. The basic ideas underlying our method and its analysis involve group representation theory. We concentrate here on the use of symmetry adapted bases and their automated generation. In this paper symmetry reduction is studied in connection with quadrature formulae and the Nyström method. A software package has been posted on the Internet.     

Equivariant functions and integrals of elliptic functions

In this paper, we introduce the theory of equivariant functions by studying their analytic, geometric and algebraic properties. We also determine the necessary and sufficient conditions under which an equivariant form arises from modular forms. This study was motivated by observing examples of functions for which the Schwarzian derivative is a modular form on a discrete group. We also investigate the Fourier expansions of normalized equivariant functions, and a strong emphasis is made on the connections to elliptic functions and their integrals.     

二维输运方程离散格式的对称性

讨论了二维柱几何非定态中子输运方程离散格式的对称性问题，在几何空间和相空间连续的情况下，证明了时间离散方程的一维球对称性；而在时间和相空间离散的情况下，阐述了格式不具有一维球对称性；对时间和相空间离散情况下的几何空间间断有限元方程，得到了左右对称性。     

A short treatise on the equivariant degree theory and its applications

The aim of this survey is to give a profound introduction to equivariant degree theory, avoiding as far as possible technical details and highly theoretical background. We describe the equivariant degree and its relation to the Brouwer degree for several classes of symmetry groups, including also the equivariant gradient degree, and particularly emphasizing the algebraic, analytical, and topological tools for its effective calculation, the latter being illustrated by six concrete examples. The paper concludes with a brief sketch of the construction and interpretation of the equivariant degree.     

On hyperbolic once-punctured-torus bundles II: fractal tessellations of the plane

We describe fractal tessellations of the complex plane that arise naturally from Cannon–Thurston maps associated to complete, hyperbolic, once-punctured-torus bundles. We determine the symmetry groups of these tessellations. To our wives, Ardyth and Elena.