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arXiv · 2610.20569

Droplets obstruct Onsager-type solutions of the Ising model in a magnetic field

Ran Luo·Jiahua Tian
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Abstract

Onsager solved the two-dimensional Ising model in zero field, but no solution is known in a magnetic field. We prove that in any dimension $d\ge2$, at every fixed dimensionless field outside a countable set, the free energy obeys no algebraic differential equation in the coupling, even locally. The obstruction arises from droplet geometry: the maximal droplet volume grows as the $d/(d-1)$ power of its boundary size, while an integer band in the pressure coefficients protects the extremal contributions against cancellation. Our result excludes Onsager-type solutions and closed forms in elliptic, hypergeometric, or Painlevé functions of the coupling at these fields.

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Ran Luo, Jiahua Tian. 2026-10-14. Droplets obstruct Onsager-type solutions of the Ising model in a magnetic field. https://arxiv.org/abs/2610.20569

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