PDE and mathematical physics · Oct 6, 2026

The exact factor-of-IID threshold for free Ising spins on trees

Determines when the free zero-field ferromagnetic Ising state on the infinite d-regular tree is a factor of independent vertex labels: exactly when tanh⁡β≤(d−1)−1/2\tanh\beta\le(d-1)^{-1/2}, including equality, for d ≥ 3 and β ≥ 0. The construction uses no root and is almost surely equivariant for each fixed tree automorphism, resolving the ferromagnetic case of Lyons's question.

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