Stochastic domination: the contact process, Ising models and FKG measures

We prove for the contact process on Zd, and many other graphs, that the upper invariant measure dominates a homogeneous product measure with large density if the infection rate λ is sufficiently large. As a consequence, this measure percolates if the corresponding product measure percolates. We raise the question of whether domination holds in the symmetric case for all infinite graphs of bounded degree. We study some asymmetric examples which we feel shed some light on this question. We next obtain necessary and sufficient conditions for domination of a product measure for “downward” FKG measures. As a consequence of this general result, we show that the plus and minus states for the Ising model on Zd dominate the same set of product measures. We show that this latter fact fails completely on the homogeneous 3-ary tree. We also provide a different distinction between Zd and the homogeneous 3-ary tree concerning stochastic domination and Ising models; while it is known that the plus states for different temperatures on Zd are never stochastically ordered, on the homogeneous 3-ary tree, almost the complete opposite is the case. Next, we show that on Zd, the set of product measures which the plus state for the Ising model dominates is strictly increasing in the temperature. Finally, we obtain a necessary and sufficient condition for a finite number of variables, which are both FKG and exchangeable, to dominate a given product measure.

RésuméOn démontre que pour les processus de contact sur Zd la mesure invariante supérieure domine une mesure produit à grande densité si le taux d'infection est suffisamment grand. Des exemples et des contre-exemples de domination sont obtenus dans divers contextes : mesures FKG, modèle d'Ising et sur différents graphes. On donne enfin une condition nécessaire et suffisante pour qu'une famille finie de variables échangeables et FKG domine une mesure produit donnée.