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Riesz Energy Subset Selection in the Euclidean Plane is NP-Hard: A Reduction from the Ising Model on Planar Cubic Graphs

Michael Emmerich

cs.CGarXiv:2608.23506

Abstract

We prove that minimum Riesz s-energy subset selection in the Euclidean plane is NP-complete already for the fixed exponent s=2. To our knowledge, this is the first Euclidean hardness result for exact Riesz-energy subset selection in which both the ambient dimension and the exponent are fixed. The reduction uses Barahona's planar cubic Ising model with uniform field. A spin is encoded by one diagonal of a four-point square. Axis-aligned selector chains implement ferromagnetic consistency, while a 45 terminal geometry yields an antiferromagnetic source interaction. Rational diagonal perturbations realize the magnetic field, and all remaining interactions are dominated by polynomial separation. Because s=2 and all coordinates are rational, every constructed energy and the decision threshold are rational exactly.

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