Analytic First Derivatives of SIDER Interpolation

Shingyu Leung

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arXiv · 2606.13998

Analytic First Derivatives of SIDER Interpolation

Shingyu Leung

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Abstract

Spherical interpolation is required in numerical and geometric applications in which the unknowns are constrained to remain on the unit sphere. Spherical Interpolation of orDER $n$ (SIDER-$n$) was introduced as the high-order reconstruction component of spherical essentially non-oscillatory interpolation, where the reconstruction is built entirely from spherical linear interpolation (SLERP) operations and therefore preserves the spherical constraint exactly. This paper develops analytic first-derivative formulas for SIDER curves of arbitrary order. The central observation is that the recursive definition of SIDER can be differentiated by direct chain-rule propagation through its binary tree of SLERP operations. After deriving the total derivative of SLERP with moving endpoints, we obtain compact recursions for the derivative of SIDER-$n$, including simplified formulas at interpolation nodes and practical formulas at middle points between consecutive sampling locations. The latter are relevant when a reconstruction is evaluated halfway between data samples, as occurs in several high-order reconstruction-based numerical algorithms. The base case SIDER2 is treated explicitly, and SIDER3 and SIDER4 are used to illustrate the recursive mechanism. We also prove that the derivative is tangent to the sphere at every reconstructed point, including both sampling points and middle points. The resulting formulas extend the original SIDER/SENO framework by supplying differential information for sphere-valued reconstructions, with potential use in high-order finite-volume, ENO/WENO, SENO-type, and related methods for conservation laws and evolution problems.

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Shingyu Leung. 2026-06-12. Analytic First Derivatives of SIDER Interpolation. https://arxiv.org/abs/2606.13998

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