# Polynomial approximation in L^p norms

Hello,

I am very new to the field of approximation theory, and since an extended search on the Internet did not provide answers for two rather basic questions, I decided to ask them here.

1) From my understanding upper bounds for

$$\inf_{q} \int_{-1}^{1} |f(x) - q(x)|^{2p} dt$$

with $f$ continuous and $q$ a polynomial of degree $n$, are expressed in terms of the $L^p$ smoothness of $f$ and in terms of the degree $n$. Could somebody point me to a proof of such a result?

2) Heuristically, what kind of information do lower bounds for the above infinum contain ? (For example, suppose that I can give a lower bound of $p!$ for the above infinimum as $p \rightarrow \infty$).

My last question might not be well-posed, so if it doesn't make sense please ignore it.

Thank you.

• It is unfortunate that you let $p$ denote both a polynomial and a positive integer. I guess that it is clear that you take the infimum over polynomials, of a certain degree? But this should be clarified. – Johan Öinert Nov 13 '11 at 1:35
• Can you clarify 2) please? Wouldn't it make more sense to study lower bounds as $n\to\infty$ when $f$ is taken to be the worst one from some function space? – timur Nov 30 '11 at 23:55

A good introductory lookup for 1) (and similar problems) is the book "Spectral Methods: Fundamentals in Single Domains" by Canuto, Hussaini, Quarteroni & Zang. Chapter 5, in particular. Equation (5.4.16) gives a bound for the $L^p$ norm approximation problem in terms of the L^p smoothness of $f$ and its derivatives: $$\inf_{q \in \mathbb{P}_n} \| f - q \| _{L^p} \leq C N^{-m} \left ( \sum^{m} _{k=\min(m,n+1)} \| f^{(k)} \|^p _{L^p} \right )^{\frac{1}{p}}$$ According to the bibliographical notes section (p.291) a proof can be found in this paper.
• There's a small typo here: the $n$'s should be either all capital or not as they all refer to the polynomial degree. – cfh Mar 25 '15 at 8:53
Results on the $L^p$-approximation theory can be found in the basic books on the subject: