The Euler-Maclaurin formula for functions with singularities

Asymptotic formulas of the Euler-Maclaurin type are proved for the sum $$\frac{1}{n}\sum\limits_{k = 1}^{n - 1} {\Phi \left( {\tfrac{k}{n}} \right) as n \to \infty .} $$ Here Φ(χ) is a sufficiently smooth function on the interval (0,1) and has singularities at the end points χ=0 and χ=1.     

Weighted approximation by Bernstein quasiinterpolants for functions with singularities

We introduce a new type of modified Bernstein quasi-interpolants, which can be used to approximate functions with singularities. We establish direct, inverse, and equivalent theorems of the weighted approximation of this modified quasi-interpolants. Some classical results on approximation of continuous functions are generalized to the weighted approximation of functions with singularities.     

Topological finite-determinacy of functions with non-isolated singularities

We introduce the concept of topological finite-determinacy for germs of analytic functions within a fixed ideal I, which provides a notion of topological finite-determinacy of functions with non-isolated singularities. We prove the following statement which generalizes classical results of Thom and Varchenko: let A be the complement in the ideal I of the space of germs whose topological type remains unchanged under a deformation within the ideal that only modifies sufficiently large order terms of the Taylor expansion. Then A has infinite codimension in I in a suitable sense. We also prove the existence of generic topological types of families of germs of I parametrized by an irreducible analytic set.     

Numerical indefinite integration of functions with singularities

We derive an indefinite quadrature formula, based on a theorem of Ganelius, for functions, for 1$">, over the interval . The main factor in the error of our indefinite quadrature formula is , with nodes and . The convergence rate of our formula is better than that of the Stenger-type formulas by a factor of in the constant of the exponential. We conjecture that our formula has the best possible value for that constant. The results of numerical examples show that our indefinite quadrature formula is better than Haber's indefinite quadrature formula for -functions.

     

The geometric complex for algebraic curves with cone-like singularities and admissible Morse functions

In a previous Note the author gave a generalisation of Witten's proof of the Morse inequalities to the model of a singular complex algebraic curve X and a stratified Morse function f. In this Note a geometric interpretation of the complex of eigenforms of the Witten Laplacian corresponding to small eigenvalues is provided in terms of an appropriate subcomplex of the complex of unstable cells of critical points of f. To cite this article: U. Ludwig, C. R. Acad. Sci. Paris, Ser. I 347 (2009).     

On a high order numerical method for functions with singularities

By splitting a given singular function into a relatively smooth part and a specially structured singular part, it is shown how the traditional Fourier method can be modified to give numerical methods of high order for calculating derivatives and integrals. Singular functions with various types of singularities of importance in applications are considered. Relations between the discrete and the continuous Fourier series for the singular functions are established. Of particular interest are piecewise smooth functions, for which various important applications are indicated, and for which numerous numerical results are presented.

     

On weighted approximation by rational operators for functions with singularities

We construct a new kind of rational operator which can be used to approximate functions with endpoints singularities by algebric weights in [?1,1], and establish new direct and converse results involving higher modulus of smoothness and a very general class of step functions, which cannot be obtained by weighted polynomial approximation. Our results also improve related results of Della Vecchia?[5].     

An extension theorem for separately holomorphic functions with pluripolar singularities

Let be a pseudoconvex domain and let be a locally pluriregular set, . Put

Let be an open neighborhood of and let be a relatively closed subset of . For let be the set of all for which the fiber is not pluripolar. Assume that are pluripolar. Put

Then there exists a relatively closed pluripolar subset of the ``envelope of holomorphy' of such that:

,

for every function separately holomorphic on there exists exactly one function holomorphic on with on , and

is singular with respect to the family of all functions .

     

Harmonic functions with a given set of singularities

Soil Physics Institute, Academy of Sciences of the USSR. Translated from Funktsional'nyi Analiz i Ego Prilozheniya, Vol. 22, No. 1, pp. 75–76, January–March, 1988.     

Removable singularities for analytic or subharmonic functions

In this paper, results on removable singularities for analytic functions, harmonic functions and subharmonic functions by Besicovitch, Carleson, and Shapiro are extended. In each theorem, we need not assume thatf has the global property at any point, so we are able to allow dense sets of singularities. We do not state our results in terms of exceptional sets, but each one leads to a series of results implying that certain sets are removable for appropriate classes of functions.     

Removable singularities for analytic or subharmonic functions

In this paper, results on removable singularities for analytic functions, harmonic functions and subharmonic functions by Besicovitch, Carleson, and Shapiro are extended. In each theorem, we need not assume thatf has the global property at any point, so we are able to allow dense sets of singularities. We do not state our results in terms of exceptional sets, but each one leads to a series of results implying that certain sets are removable for appropriate classes of functions. Partially supported by an NSF-Grant and an XL-Grant at Purdue respectively.     

Differential algebras of generalised functions with dense singularities

It is shown how chains of algebras of generalised functions may be used to construct algebras of generalised functions that are able to deal with larger classes of singularities than each of the constituent algebras in the chain. The general method is applied to a chain of almost everywhere algebras, yielding an algebra that can handle certain densely singular functions. The embedding of the distributions into the mentioned algebra, as well as the existence of solutions of nonlinear PDEs, is considered.     

Interpolation of analytic functions with finitely many singularities

We consider an interpolation process for the class of functions with finitely many singular points by means of rational functions whose poles coincide with the singular points of the function under interpolation. The interpolation nodes form a triangular matrix. We find necessary and sufficient conditions for the uniform convergence of sequences of interpolation fractions to the function under interpolation on every compact set disjoint from the singular points of the function and other conditions for convergence. We generalize and improve the familiar results on the interpolation of functions with finitely many singular points by rational fractions and of entire functions by polynomials.     

Optimal adaptive solution of initial-value problems with unknown singularities

The optimal solution of initial-value problems in ODEs is well studied for smooth right-hand side functions. Much less is known about the optimality of algorithms for singular problems. In this paper, we study the (worst case) solution of scalar problems with a right-hand side function having r   continuous bounded derivatives in R$\mathbf{R}$, except for an unknown singular point. We establish the minimal worst case error for such problems (which depends on r similarly as in the smooth case), and define optimal adaptive algorithms. The crucial point is locating an unknown singularity of the solution by properly adapting the grid. We also study lower bounds on the error of an algorithm for classes of singular problems. In the case of a single singularity with nonadaptive information, or in the case of two or more singularities, the error of any algorithm is shown to be independent of r.     

The Witten deformation for even dimensional spaces with cone-like singularities and admissible Morse functions

In this Note we generalise the Witten deformation to even dimensional Riemannian manifolds with cone-like singularities X and certain functions f, which we call admissible Morse functions. As a corollary we get Morse inequalities for the L²-Betti numbers of X. The contribution of a singular point p of X to the Morse inequalities can be expressed in terms of the intersection cohomology of the local Morse datum of f at p. The definition of the class of functions which we study here is inspired by stratified Morse theory as developed by Goresky and MacPherson. However the setting here is different since the spaces considered here are manifolds with cone-like singularities instead of Whitney stratified spaces.