Uniform and pointwise shape preserving approximation (SPA) by algebraic polynomials: an update

Abstract

It is not surprising that one should expect that the degree of constrained (shape preserving) approximation be worse than the degree of unconstrained approximation. However, it turns out that, in certain cases, these degrees are the same. The main purpose of this paper is to provide an update to our 2011 survey paper. In particular, we discuss recent uniform estimates in comonotone approximation, mention recent developments and state several open problems in the (co)convex case, and reiterate that co-$q$-monotone approximation with $q\ge 3$ is completely different from comonotone and coconvex cases. Additionally, we show that, for each function $f$ from $\Delta^{(1)}$, the set of all monotone functions on $[-1,1]$, and every $\alpha>0$, we have $$\limsup_{n\to\infty} \inf_{P_n\in\mathbb P_n\cap\Delta^{(1)}} \left\| \frac{n^\alpha(f-P_n)}{\varphi^\alpha} \right\| \le c(\alpha) \limsup_{n\to\infty} \inf_{P_n\in\mathbb P_n} \left\| \frac{n^\alpha(f-P_n)}{\varphi^\alpha} \right\|$$ where $\mathbb P_n$ denotes the set of algebraic polynomials of degree $<n$, $\varphi(x):=\sqrt{1-x^2}$, and $c=c(\alpha)$ depends only on $\alpha$