Self-conjugate-grid pointwise interpolation-error conjecture

Let D\mathbb{D} be the unit disc, let En(D)\mathscr{E}_n(\mathbb{D}) be the entire functions g:CCg:\mathbb{C}\to\mathbb{C} satisfying g(n)D1\|g^{(n)}\|_{\mathbb{D}}\leqslant1, and let z1:nDnz_{1:n}\in\mathbb{D}^n be self-conjugate, meaning that nonreal nodes occur with their conjugates and every real node occurs with even multiplicity. Let f(z1:n)f(\cdot\mid z_{1:n}) be the nodal polynomial and g^(z1:n)\hat g(\cdot\mid z_{1:n}) the interpolation polynomial.

Self-conjugate-grid pointwise conjecture. For every 0kn10\leqslant k\leqslant n-1,

supgEn(D)g(k)(z)g^(k)(zz1:n)z=1=1n!f(k)(zz1:n)z=1.\left.\operatorname{\mathsf{\sup}}_{g\in\mathscr{E}_n(\mathbb{D})}|g^{(k)}(z)-\hat g^{(k)}(z\mid z_{1:n})|\right|_{z=1}=\left.\frac{1}{n!}|f^{(k)}(z\mid z_{1:n})|\right|_{z=1}.

If additionally Re(z1:n)R+n\operatorname{\mathsf{Re}}(z_{1:n})\in\mathbb{R}^n_+, the same equality holds at z=0z=0. This is a proposed extension of the pointwise worst-case interpolation result from real nodes to self-conjugate grids; its resolution is not supplied in the source.

Sources & referencesView supporting material

Primary source

Dmitrii M. Ostrovskii and Pavel S. Shcherbakov, “Amplitude maximization in stable systems, Schur positivity, and some conjectures on polynomial interpolation”, arXiv:2508.13554 (2025).

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