Complex Eigenvalue Splitting for the Dirac Operator

We analyze the eigenvalue problem for the semiclassical Dirac (or Zakharov–Shabat) operator on the real line with general analytic potential. We provide Bohr–Sommerfeld quantization conditions near energy levels where the potential exhibits the characteristics of a single or double bump function. From these conditions we infer that near energy levels where the potential (or rather its square) looks like a single bump function, all eigenvalues are purely imaginary. For even or odd potentials we infer that near energy levels where the square of the potential looks like a double bump function, eigenvalues split in pairs exponentially close to reference points on the imaginary axis. For even potentials this splitting is vertical and for odd potentials it is horizontal, meaning that all such eigenvalues are purely imaginary when the potential is even, and no such eigenvalue is purely imaginary when the potential is odd.

Fourier Series Windowed by a Bump Function

We study the Fourier transform windowed by a bump function. We transfer Jackson’s classical results on the convergence of the Fourier series of a periodic function to windowed series of a not necessarily periodic function. Numerical experiments illustrate the obtained theoretical results.

Comparison between W2 distance and Ḣ−1 norm, and Localization of Wasserstein distance

It is well known that the quadratic Wasserstein distance W2(⋅, ⋅) is formally equivalent, for infinitesimally small perturbations, to some weighted H−1 homogeneous Sobolev norm. In this article I show that this equivalence can be integrated to get non-asymptotic comparison results between these distances. Then I give an application of these results to prove that the W2 distance exhibits some localization phenomenon: if μ and ν are measures on ℝn and ϕ: ℝn → ℝ+ is some bump function with compact support, then under mild hypotheses, you can bound above the Wasserstein distance between ϕ ⋅ μ and ϕ ⋅ ν by an explicit multiple of W2(μ, ν).

Porosity, Γ‎N- and Γ‎-Null Sets

This chapter introduces the notion of porosity “at infinity” (formally defined as porosity with respect to a family of subspaces) and discusses the main result, which shows that sets porous with respect to a family of subspaces are Γ‎ₙ-null provided X admits a continuous bump function whose modulus of smoothness (in the direction of this family) is controlled by tⁿ logⁿ⁻¹ (1/t). The first of these results characterizes Asplund spaces: it is shown that a separable space has separable dual if and only if all its porous sets are Γ‎₁-null. The chapter first describes porous and σ‎-porous sets as well as a criterion of Γ‎ₙ-nullness of porous sets. It then considers the link between directional porosity and Γ‎ₙ-nullness. Finally, it tackles the question in which spaces, and for what values of n, porous sets are Γ‎ₙ-null.

Bump Functions with Hölder Derivatives

We study the range of the gradients of aC1,α-smooth bump function defined on a Banach space. We find that this set must satisfy two geometrical conditions: It can not be too flat and it satisfies a strong compactness condition with respect to an appropriate distance. These notions are defined precisely below. With these results we illustrate the differences with the case ofC1-smooth bump functions. Finally, we give a sufficient condition on a subset ofX* so that it is the set of the gradients of aC1,1-smooth bump function. In particular, ifXis an infinite dimensional Banach space with aC1,1-smooth bump function, then any convex open bounded subset ofX* containing 0 is the set of the gradients of aC1,1-smooth bump function.

ON THE ACCURACY OF SURFACE SPLINE APPROXIMATION AND INTERPOLATION TO BUMP FUNCTIONS

Let $\sOm$ be the closure of a bounded open set in $\mathbb{R}^d$, and, for a sufficiently large integer $\kappa$, let $f\in C^\kappa(\sOm)$ be a real-valued ‘bump’ function, i.e. $\supp(f)\subset\textint(\sOm)$. First, for each $h>0$, we construct a surface spline function $\sigma_h$ whose centres are the vertices of the grid $\mathcal{V}_h=\sOm\cap h\zd$, such that $\sigma_h$ approximates $f$ uniformly over $\sOm$ with the maximal asymptotic accuracy rate for $h\rightarrow0$. Second, if $\ell_1,\ell_2,\dots,\ell_n$ are the Lagrange functions for surface spline interpolation on the grid $\mathcal{V}_h$, we prove that $\max_{x\in\sOm}\sum_{j=1}^n\ell_j^2(x)$ is bounded above independently of the mesh-size $h$. An interesting consequence of these two results for the case of interpolation on $\mathcal{V}_h$ to the values of a bump data function $f$ is obtained by means of the Lebesgue inequality.AMS 2000 Mathematics subject classification: Primary 41A05; 41A15; 41A25; 41A63

Smooth Partitions of Unity on Banach Spaces

It is shown that if a Banach space X admits a Ck-smooth bump function, and X* is Asplund, then X admits Ck-smooth partitions of unity.

A note on bump functions that locally depend on finitely many coordinates

We show that if a continuous bump function on a Banach space X locally depends on finitely many elements of a set F in X*, then the norm closed linear span of F equals to X*. Some corollaries for Markuševič bases and Asplund spaces are derived.