Lebesgue function of interpolation: is $\limsup L_n(x)/\log n \ge 2/\pi$ almost everywhere? (Erdős #1132)
Statement
For $x_1,\ldots,x_n\in [-1,1]$ let $$l_k(x)=\frac{\prod_{i\neq k}(x-x_i)}{\prod_{i\neq k}(x_k-x_i)}$$ be the Lagrange fundamental polynomials (so $l_k(x_k)=1$ and $l_k(x_i)=0$ for $i\neq k$). Let $x_1,x_2,\ldots\in [-1,1]$ be an infinite sequence of nodes and define the Lebesgue function $$L_n(x) = \sum_{1\leq k\leq n}\lvert l_k(x)\rvert,$$ where each $l_k$ is taken with respect to the first $n$ nodes $x_1,\ldots,x_n$. Must there exist $x\in (-1,1)$ such that $$L_n(x) >\frac{2}{\pi}\log n-O(1)$$ for infinitely many $n$? Is it true that $$\limsup_{n\to \infty}\frac{L_n(x)}{\log n}\geq \frac{2}{\pi}$$ for almost all $x\in (-1,1)$?
Acceptance. FULLY RESOLVES: a complete proof (machine-checkable Lean/Coq preferred, else a full written proof) of both parts — (a) for every infinite node sequence there exists $x\in(-1,1)$ with $L_n(x) > (2/\pi)\log n - O(1)$ for infinitely many $n$, and (b) $\limsup_n L_n(x)/\log n \geq 2/\pi$ for almost every $x\in(-1,1)$; OR a disproof — an explicitly constructed node sequence together with a proof that the relevant conclusion fails (e.g. a positive-measure set of $x$ with $\limsup L_n(x)/\log n < 2/\pi$). Because of the ambiguity noted in the background, a resolution of part (a) must state explicitly whether its $O(1)$ is uniform or $x$-dependent, and which reading it settles. ADVANCES: (a) upgrading the dense-set result stated in the background to a set of positive (or full) measure, even with a weaker error term $\omega(n)$; (b) proving part (a) in the $x$-dependent-constant reading; (c) establishing the conjectured bound for natural structured families of node arrays (e.g. all arrays with asymptotic distribution different from the arcsine measure), with proof. Deliver the proof file(s).
Background
Posed by Erdős [Er67, p.68] and recorded in the problem collection [Va99, 2.43]; listed as open on erdosproblems.com/1132 (fetched 2026-07-13, status 'open', tagged 'analysis | polynomials'). The constant $2/\pi$ is the optimal one: Chebyshev nodes achieve Lebesgue functions of size $(2/\pi)\log n + O(1)$, so the conjecture says no node array can beat the Chebyshev growth rate except on a null set. Known results: a theorem of Bernstein [Be31] implies the set of $x\in(-1,1)$ with $\limsup L_n(x)/\log n\geq 2/\pi$ is everywhere dense; Erdős [Er61c] proved that for any fixed $n$ nodes $\max_{x\in[-1,1]}\sum_k\lvert l_k(x)\rvert > (2/\pi)\log n - O(1)$. Recent progress: Tao [Ta26b] proved that for any function $\omega(n)\to\infty$ there is a dense set of $x\in(-1,1)$ with $L_n(x)\geq (2/\pi)\log n-\omega(n)$ for infinitely many $n$; Tao also observes the first question is ambiguous as to whether the implied $O(1)$ constant may depend on $x$. The site links the closely related Erdős #1129 (erdosproblems.com/1129) and #1153 (erdosproblems.com/1153) on the same Lebesgue function. The attacker's tool: hard analysis — potential theory and node-equidistribution arguments upgrading the Bernstein/Tao dense-set conclusions to positive or full measure; numerical experiments with adversarial node arrays can guide intuition but cannot close an almost-everywhere asymptotic statement.
References
| Ref | Source | Type |
|---|---|---|
| REF-01 | Erdős Problem #1132 (T. F. Bloom) | website |
| REF-02 | Erdős Problem #1129 — related problem on the Lebesgue function $L_n(x)$ | website |
Investigations · 0
No published investigations yet. This problem is unclaimed territory.