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Erdős problem 671

For triangular arrays of nodes ain∈[−1,1]a_i^n\in[-1,1] let Lnf\mathcal{L}^nf be the Lagrange interpolation polynomials of a continuous ff, with fundamental polynomials pinp_i^n. Is there a choice of nodes such that for every continuous ff there is some xx where lim sup⁡n∑i∣pin(x)∣=∞\limsup_n \sum_i\lvert p_{i}^n(x)\rvert=\infty and yet Lnf(x)→f(x)\mathcal{L}^nf(x) \to f(x)? Is there a choice with lim sup⁡n∑i∣pin(x)∣=∞\limsup_n \sum_i\lvert p_{i}^n(x)\rvert=\infty for every xx, yet for every continuous ff some xx has Lnf(x)→f(x)\mathcal{L}^nf(x)\to f(x)? Both questions are claimed resolved in the affirmative.

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For triangular arrays of nodes $a_i^n\in[-1,1]$ let $\mathcal{L}^nf$ be the Lagrange interpolation polynomials of a continuous $f$, with fundamental polynomials $p_i^n$. Is there a choice of nodes such that for every continuous $f$ there is some $x$ where $\limsup_n \sum_i\lvert p_{i}^n(x)\rvert=\infty$ and yet $\mathcal{L}^nf(x) \to f(x)$? Is there a choice with $\limsup_n \sum_i\lvert p_{i}^n(x)\rvert=\infty$ for every $x$, yet for every continuous $f$ some $x$ has $\mathcal{L}^nf(x)\to f(x)$? Both questions are claimed resolved in the affirmative.

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