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arXiv · 2608.17920

Antiferromagnetic models are clique-minimizing

Joonkyung Lee·Jaehyeon Seo
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Abstract

An edge-weighted graph $H$, possibly with loops, is antiferromagnetic if its adjacency matrix is entrywise nonnegative and has at most one positive eigenvalue, counted with multiplicity. We show that, for any graph $G$ with $d_v:=\operatorname{deg}_G(v)$, $$\operatorname{hom}(G,H) \ge \prod_{v\in V(G)} \operatorname{hom}(K_{d_v+1},H)^{\frac{1}{d_v+1}},$$ whenever $H$ is antiferromagnetic. In fact, we prove a vertex-inhomogeneous strengthening of this inequality, allowing a different fugacity vector at each vertex of $G$. This gives a common generalization of the lower-bound inequalities of Sah, Sawhney, Stoner, and Zhao for independent sets, of Csikv\'{a}ri for $q$-colorings, and of the authors for semiproper colorings with at most two proper colors. Furthermore, it confirms recent conjectures of the authors and of Davies and LeBlanc. A key ingredient, of independent interest, is a strengthening of the delete-one form of Shearer's inequality for Lorentzian measures, which provides a new approach to graph homomorphism inequalities.

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Joonkyung Lee, Jaehyeon Seo. 2026-08-18. Antiferromagnetic models are clique-minimizing. https://arxiv.org/abs/2608.17920

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