arXiv subjects

Dany Leviatan

Publications and source records attributed to Dany Leviatan.

6 recordsLinked to original sources

arXiv2021

Exact pointwise estimates for polynomial approximation with Hermite interpolation

We establish best possible pointwise (up to a constant multiple) estimates for approximation, on a finite interval, by polynomials that satisfy finitely many (Hermite) interpolation conditions, and show that these estimates cannot be improved. In particular, we show that {\bf any} algebraic polynomial of degree $n$ approximating a function $f\in C^r(I)$, $I=[-1,1]$, at the classical pointwise rate $ρ_n^r(x) ω_k(f^{(r)}, ρ_n(x))$, where $ρ_n(x)=n^{-1}\sqrt{1-x^2}+n^{-2}$, and (Hermite) interpolating $f$ and its derivatives up to the order $r$ at a point $x_0\in I$, has the best possible pointwise rate of (simultaneous) approximation of $f$ near $x_0$. Several applications are given.

math.CA↗

arXiv2020

No Jackson-type estimates for piecewise $q$-monotone, $q\ge3$, trigonometric approximation

We say that a function $f\in C[a,b]$ is $q$-monotone, $q\ge3$, if $f\in C^{q-2}(a,b)$ and $f^{(q-2)}$ is convex in $(a,b)$. Let $f$ be continuous and $2π$-periodic, and change its $q$-monotonicity finitely many times in $[-π,π]$. We are interested in estimating the degree of approximation of $f$ by trigonometric polynomials which are co-$q$-monotone with it, namely, trigonometric polynomials that change their $q$-monotonicity exactly at the points where $f$ does. Such Jackson type estimates are valid for piecewise monotone ($q=1$) and piecewise convex (q=2) approximations. However, we prove, that no such estimates are valid, in general, for co-$q$-monotone approximation, when $q\ge3$.

math.CA↗

arXiv2018

On moduli of smoothness with Jacobi weights

The main purpose of this paper is to introduce moduli of smoothness with Jacobi weights $(1-x)^α(1+x)^β$ for functions in the Jacobi weighted $L_p[-1,1]$, $0<p\le \infty$, spaces. These moduli are used to characterize the smoothness of (the derivatives of) functions in the weighted $L_p$ spaces. If $1\le p\le\infty$, then these moduli are equivalent to certain weighted $K$-functionals (and so they are equivalent to certain weighted Ditzian-Totik moduli of smoothness for these $p$), while for $0<p<1$ they are equivalent to certain "Realization functionals".

math.CA↗

arXiv2018

Interpolatory estimates for convex piecewise polynomial approximation

In this paper, among other things, we show that, given $r\in N$, there is a constant $c=c(r)$ such that if $f\in C^r[-1,1]$ is convex, then there is a number ${\mathcal N}={\mathcal N}(f,r)$, depending on $f$ and $r$, such that for $n\ge{\mathcal N}$, there are convex piecewise polynomials $S$ of order $r+2$ with knots at the Chebyshev partition, satisfying \[ |f(x)-S(x)|\le c(r)\left( \min\left\{ 1-x^2, n^{-1}\sqrt{1-x^2} \right\} \right)^r ω_2\left(f^{(r)}, n^{-1}\sqrt{1-x^2} \right), \] for all $x\in [-1,1]$. Moreover, ${\mathcal N}$ cannot be made independent of $f$.

math.CA↗

arXiv2018

On the stability and accuracy of least squares approximations

We consider the problem of reconstructing an unknown function $f$ on a domain $X$ from samples of $f$ at $n$ randomly chosen points with respect to a given measure $ρ_X$. Given a sequence of linear spaces $(V_m)_{m>0}$ with ${\rm dim}(V_m)=m\leq n$, we study the least squares approximations from the spaces $V_m$. It is well known that such approximations can be inaccurate when $m$ is too close to $n$, even when the samples are noiseless. Our main result provides a criterion on $m$ that describes the needed amount of regularization to ensure that the least squares method is stable and that its accuracy, measured in $L^2(X,ρ_X)$, is comparable to the best approximation error of $f$ by elements from $V_m$. We illustrate this criterion for various approximation schemes, such as trigonometric polynomials, with $ρ_X$ being the uniform measure, and algebraic polynomials, with $ρ_X$ being either the uniform or Chebyshev measure. For such examples we also prove similar stability results using deterministic samples that are equispaced with respect to these measures.

math.NA↗

arXiv2017

On weighted approximation with Jacobi weights

We obtain matching direct and inverse theorems for the degree of weighted $L_p$-approximation by polynomials with the Jacobi weights $(1-x)^α(1+x)^β$. Combined, the estimates yield a constructive characterization of various smoothness classes of functions via the degree of their approximation by algebraic polynomials.

math.CA↗