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arXiv · 2507.20482

Cutoff for the Swendsen-Wang dynamics on the complete graph

Antonio Blanca·Zhezheng Song
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Abstract

We study the speed of convergence of the Swendsen-Wang (SW) dynamics for the $q$-state ferromagnetic Potts model on the $n$-vertex complete graph, known as the mean-field model. The SW dynamics was introduced as an attractive alternative to the local Glauber dynamics, often offering faster convergence rates to stationarity in a variety of settings. A series of works have characterized the asymptotic behavior of the speed of convergence of the mean-field SW dynamics for all $q \ge 2$ and all values of the inverse temperature parameter $β> 0$. In particular, it is known that when $β> q$ the mixing time of the SW dynamics is $Θ(\log n)$. We strengthen this result by showing that for all $β> q$, there exists a constant $c(β,q) > 0$ such that the mixing time of the SW dynamics is $c(β,q) \log n + Θ(1)$. This implies that the mean-field SW dynamics exhibits the cutoff phenomenon in this temperature regime, demonstrating that this Markov chain undergoes a sharp transition from ''far from stationarity'' to ''well-mixed'' within a narrow $Θ(1)$ time window. The presence of cutoff is algorithmically significant, as simulating the chain for fewer steps than its mixing time could lead to highly biased samples.

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Antonio Blanca, Zhezheng Song. 2025-10-13. Cutoff for the Swendsen-Wang dynamics on the complete graph. https://arxiv.org/abs/2507.20482

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