We explore numerically the eigenvalues of the hermitian form$$\sum _{q\le Q}\sum _{amod{}^{*}q}{\left|\sum _{n\le N}{\varphi }_{n}e(na/q)\right|}^2$$when $N={\sum }_{q\le Q}\phi \left(q\right)$. We improve on the existing upper bound, and produce a (conjectural) plot of the asymptotic distribution of its eigenvalues by exploiting fairly extensive computations. The main outcome is that this asymptotic density most probably exists but is not continuous with respect to the Lebesgue measure.

Suppose Δ̃ is the Laplace-Beltrami operator on the sphere $S^{d-1},{\Delta }_{\rho }^{k}f\left(x\right)={\Delta }_{\rho }{\Delta }_{\rho }^{k-1}f\left(x\right)$ and ${\Delta }_{\rho }f\left(x\right)=f\left(\rho x\right)-f\left(x\right)$ where ρ ∈ SO(d). Then ${\omega }^m{(f,t)}_{L_p\left(S^{d-1}\right)}\equiv sup\parallel {\Delta }_{\rho }^{m}f{\parallel }_{L_p\left(S^{d-1}\right)}:\rho \in SO\left(d\right),ma{x}_{x\in S^{d-1}}\rho x·x\ge cost$ and $K̃ₘ{(f,t^m)}_p\equiv inf\parallel f-g{\parallel }_{L_p\left(S^{d-1}\right)}+t^m\parallel {(-\Delta ̃)}^{m/2}g{\parallel }_{L_p\left(S^{d-1}\right)}:g\in \left({(-\Delta ̃)}^{m/2}\right)$ are equivalent for 1 < p < ∞. We note that for even m the relation was recently investigated by the second author. The equivalence yields an extension of the results on sharp Jackson inequalities on the sphere. A new strong converse inequality for $L_p\left(S^{d-1}\right)$ given in this paper plays a significant role in the proof.

Interest in meshfree methods in solving boundary-value problems has grown rapidly in recent years. A meshless method that has attracted considerable interest in the community of computational mechanics is built around the idea of modified local Shepard’s partition of unity. For these kinds of applications it is fundamental to analyze the order of the approximation in the context of Sobolev spaces. In this paper, we study two different techniques for building modified local Shepard’s formulas, and...

Interest in meshfree methods in solving boundary-value problems has grown rapidly in recent years. A meshless method that has attracted considerable interest in the community of computational mechanics is built around the idea of modified local Shepard's partition of unity. For these kinds of applications it is fundamental to analyze the order of the approximation in the context of Sobolev spaces. In this paper, we study two different techniques for building modified local Shepard's formulas, and...

Our aim is to estimate the joint distribution of a finite sequence of independent categorical variables. We consider the collection of partitions into dyadic intervals and the associated histograms, and we select from the data the best histogram by minimizing a penalized least-squares criterion. The choice of the collection of partitions is inspired from approximation results due to DeVore and Yu. Our estimator satisfies a nonasymptotic oracle-type inequality and adaptivity properties in the minimax...

Our aim is to estimate the joint distribution of a finite sequence of independent categorical variables. We consider the collection of partitions into dyadic intervals and the associated histograms, and we select from the data the best histogram by minimizing a penalized least-squares criterion. The choice of the collection of partitions is inspired from approximation results due to DeVore and Yu. Our estimator satisfies a nonasymptotic oracle-type inequality and adaptivity properties in the minimax...

Given an equibounded (₀)-semigroup of linear operators with generator A on a Banach space X, a functional calculus, due to L. Schwartz, is briefly sketched to explain fractional powers of A. Then the (modified) K-functional with respect to $(X,D\left({(-A)}^{\alpha }\right))$, α > 0, is characterized via the associated resolvent R(λ;A). Under the assumption that the resolvent satisfies a Nikolskii type inequality, ${\left|\right|\lambda R(\lambda ;A)f\left|\right|}_Y{\le c\phi (1/\lambda )\left|\right|f\left|\right|}_X$, for a suitable Banach space Y, an Ulyanov inequality is derived. This will be of interest if one has good control...

We study the uniform approximation properties of general multivariate singular integral operators on ${ℝ}^N$, N ≥ 1. We establish their convergence to the unit operator with rates. The estimates are pointwise and uniform. The established inequalities involve the multivariate higher order modulus of smoothness. We list the multivariate Picard, Gauss-Weierstrass, Poisson-Cauchy and trigonometric singular integral operators to which this theory can be applied directly.

We point out relations between Siciak’s homogeneous extremal function ${\Psi }_B$ and the Cauchy-Poisson transform in case $B$ is a ball in ℝ². In particular, we find effective formulas for ${\Psi }_B$ for an important class of balls. These formulas imply that, in general, ${\Psi }_B$ is not a norm in ℂ².

We prove that every singular algebraic curve in ℝⁿ admits local tangential Markov inequalities at each of its points. More precisely, we show that the Markov exponent at a point of a real algebraic curve A is less than or equal to twice the multiplicity of the smallest complex algebraic curve containing A.