## Electronic Journal of Probability

### On Disagreement Percolation and Maximality of the Critical Value for iid Percolation

Johan Jonasson

#### Abstract

Two different problems are studied:

• 1. For an infinite locally finite connected graph $G$, let $p_c(G)$ be the critical value for the existence of an infinite cluster in iid bond percolation on $G$ and let $P_c = \sup\{p_c(G): G \text{ transitive }, p_c(G) \lt 1\}$. Is $P_c \lt 1$?
• 2. Let $G$ be transitive with $p_c(G) \lt 1$, take $p \in [0,1]$ and let $X$ and $Y$ be iid bond percolations on $G$ with retention parameters $(1+p)/2$ and $(1-p)/2$ respectively. Is there a $q \lt 1$ such that $p \gt q$ implies that for any monotone coupling $(X',Y')$ of $X$ and $Y$ the edges for which $X'$ and $Y'$ disagree form infinite connected component(s) with positive probability? Let $p_d(G)$ be the infimum of such $q$'s (including $q=1$) and let $P_d = \sup\{p_d(G): G \text{ transitive }, p_c(G) \lt 1\}$. Is the stronger statement $P_d \lt 1$ true? On the other hand: Is it always true that $p_d(G) \gt p_c (G)$?

It is shown that if one restricts attention to biregular planar graphs then these two problems can be treated in a similar way and all the above questions are positively answered. We also give examples to show that if one drops the assumption of transitivity, then the answer to the above two questions is no. Furthermore it is shown that for any bounded-degree bipartite graph $G$ with $p_c(G) \lt 1$ one has $p_c(G) \lt p_d(G)$. Problem (2) arises naturally from [6] where an example is given of a coupling of the distinct plus- and minus measures for the Ising model on a quasi-transitive graph at super-critical inverse temperature. We give an example of such a coupling on the $r$-regular tree, ${\bf T}_r$, for $r \gt 1$.

#### Article information

Source
Electron. J. Probab., Volume 6 (2001), paper no. 15, 13 pp.

Dates
Accepted: 15 June 2001
First available in Project Euclid: 19 April 2016

https://projecteuclid.org/euclid.ejp/1461097645

Digital Object Identifier
doi:10.1214/EJP.v6-88

Mathematical Reviews number (MathSciNet)
MR1873292

Zentralblatt MATH identifier
0987.60099

Rights

#### Citation

Jonasson, Johan. On Disagreement Percolation and Maximality of the Critical Value for iid Percolation. Electron. J. Probab. 6 (2001), paper no. 15, 13 pp. doi:10.1214/EJP.v6-88. https://projecteuclid.org/euclid.ejp/1461097645

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