Norming Sets and Related Remez-type Inequalities

The classical Remez inequality bounds the maximum of the absolute value of a real polynomial $P$ of degree $d$ on $[-1,1]$ through the maximum of its absolute value on any subset $Z\subset [-1,1]$ of positive Lebesgue measure. Extensions to several variables and to certain sets of Lebesgue measure zero, massive in a much weaker sense, are available. Still, given a subset $Z\subset [-1,1]^n\subset {\mathbb R}^n$ it is not easy to determine whether it is ${\mathcal P}_d({\mathbb R}^n)$-norming (here ${\mathcal P}_d({\mathbb R}^n)$ is the space of real polynomials of degree at most $d$ on ${\mathbb R}^n$), i.e. satisfies a Remez-type inequality: $\sup_{[-1,1]^n}|P|\le C\sup_{Z}|P|$ for all $P\in {\mathcal P}_d({\mathbb R}^n)$ with $C$ independent of $P$. (Although ${\mathcal P}_d({\mathbb R}^n)$-norming sets are exactly those not contained in any algebraic hypersurface of degree $d$ in ${\mathbb R}^n$, there are many apparently unrelated reasons for $Z \subset [-1,1]^n$ to have this property.) In the present paper we study norming sets and related Remez-type inequalities in a general setting of finite-dimensional linear spaces $V$ of continuous functions on $[-1,1]^n$, remaining in most of the examples in the classical framework. First, we discuss some sufficient conditions for $Z$ to be $V$-norming, partly known, partly new, restricting ourselves to the simplest non-trivial examples. Next, we extend the Turan-Nazarov inequality for exponential polynomials to several variables, and on this base prove a new fewnomial Remez-type inequality. Finally, we study the family of optimal constants $N_{V}(Z)$ in the Remez-type inequalities for $V$, as the function of the set $Z$, showing that it is Lipschitz in the Hausdorff metric.

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