Uniform interpolation-error conjecture for self-conjugate grids

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Let II be the interpolation interval, let D\mathbb{D} be the unit disc, let En(D)\mathscr{E}_n(\mathbb{D}) be the entire functions g:C→Cg:\mathbb{C}\to\mathbb{C} satisfying ∥g(n)∥D⩽1\|g^{(n)}\|_{\mathbb{D}}\leqslant1, and let z1:n∈Dnz_{1:n}\in\mathbb{D}^n be a self-conjugate grid. 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 uniform conjecture. For every 0⩽k⩽n−10\leqslant k\leqslant n-1,

sup⁡⁡g∈En(D)∥g(k)(⋅)−g^(k)(⋅∣z1:n)∥I=1n!∥f(k)(⋅∣z1:n)∥I.\operatorname{\mathsf{\sup}}_{g\in\mathscr{E}_n(\mathbb{D})}\|g^{(k)}(\cdot)-\hat g^{(k)}(\cdot\mid z_{1:n})\|_I=\frac{1}{n!}\|f^{(k)}(\cdot\mid z_{1:n})\|_I.

Equivalently, the source gives the corresponding supremum identity involving Schur polynomials. This would generalize the real-node result of Kallioniemi and Shadrin to self-conjugate grids in the unit disc; the source does not provide a resolution.

References

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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