Summability of eigenfunction expansions of differential and integral operators

Translated from Matematicheskie Zametki, Vol. 54, No. 3, pp. 114–122, September, 1993.     

Pointwise fourier inversion and related eigenfunction expansions

Necessary and sufficient conditions are found for the convergence at a pre-assigned point of the spherical partial sums (resp. integrals) of the Fourier series (resp. integral) in the class of piecewise smooth functions on Euclidean space. These results carry over unchanged to spherical harmonic expansions, Fourier transforms on hyperbolic space, and Dirichlet eigenfunction expansions with respect to the Laplace operator on a class of Riemannian manifolds. © 1994 John Wiley & Sons, Inc.     

Localization and convergence of eigenfunction expansions

We state a localization principle for expansions in eigenfunctions of a self-adjoint second order elliptic operator and we prove an equiconvergence result between eigenfunction expansions and trigonometric expansions. We then study the Gibbs phenomenon for eigenfunction expansions of piecewise smooth functions on two-dimensional manifolds.     

Riesz means of eigenfunction expansions of elliptic differential operators on compact manifolds

Let {λ ²} and {? _λ } be the eigenvalues and an orthonormal system of eigenvectors of a second order elliptic differential operator Δ on a compact manifoldM of dimensionN. We prove that the Riesz means of order δ, defined by \(R_\Lambda ^\delta f = \sum\limits_{\lambda< \Lambda } {\left( {1 - \frac{{\lambda ^2 }}{{\Lambda ^2 }}} \right)^\delta \hat f(} \lambda ) \varphi _\lambda \) , are uniformly bounded from the Hardy spaceH ^p (M) into Weak-L ^p (M), if 0<p<1 and δ=N/p?(N+1)/2.     

Pointwise lower and upper bounds for eigenfunctions of ordinary differential operators

Summary For a differential operatorL as defined in (2.1) we consider the eigenvalue problemL u=u and describe a method to obtain a pointwise bound |u–v| wherev denotes the Rayleigh-Ritz approximation to an exact eigenfunctionu. The upper bound is continuous, but only piecewise differentiable and calculated by solving certain (inverse-positive!) auxiliary problems.Our method uses well-known estimations for (t _i) at a finite number of pointst _i[a, b] to calculate an upper, bound in the whole interval [a, b].The author would like to, thank the Battelle Institute Geneve for their assistance     

Absolute convergence of eigenfunction expansions of integral operators with kernels admitting discontinuities of derivatives on diagonals

In this paper, for expansions in eigenfunctions and associated functions, an analog of the known A. Zygmund theorem on the absolute convergence of trigonometric Fourier series is proved. __________ Translated from Sovremennaya Matematika i Ee Prilozheniya (Contemporary Mathematics and Its Applications), Vol. 33, Suzdal Conference-2004, Part 1, 2005.     

Absolute and uniform convergence of eigenfunction expansions of integral operators with kernels admitting derivative discontinuities on the diagonals

An analog of Szasz’s theorem on the absolute convergence of trigonometric Fourier series is established for expansions in the eigen and associated functions of integral operators some of whose kernels involve derivatives with discontinuities on the diagonals.     

Pointwise convergence of expansions with respect to certain product systems

- , - _n A_n-,. , . , ¦_n¦=1 (n=1,2, ), . , — — , .     

Pointwise convergence of a class of non-orthogonal wavelet expansions

Non-orthogonal wavelet expansions associated with a class of mother wavelets is considered. This class of wavelets comprises mother wavelets that are not necessarily integrable over the whole real line, such as Shannon's wavelet. The pointwise convergence of these wavelet expansions is investigated. It is shown that, unlike other wavelet expansions, the ones under consideration do not necessarily converge pointwise to the functions at points of continuity, unless a more stringent condition, such as bounded variation, is imposed.