# On the two-point function of the Ising model with infinite range-interactions

## TL;DR

This paper investigates the behavior of the two-point function in the infinite-range Ising model, establishing new results on its divergence, asymptotics, and correlation length at critical parameters.

## Contribution

It provides the first analysis of the two-point function at the saturation inverse correlation length in the infinite-range Ising model.

## Key findings

- Laplace transform of the two-point function is infinite at critical saturation point.
- Two-point function exhibits Ornstein-Zernike asymptotics at the saturation point.
- Existence of a parameter range where the inverse correlation length equals the decay rate.

## Abstract

In this article, we prove some results concerning the truncated two-point function of the infinite-range Ising model above and below the critical temperature. More precisely, if the coupling constants are of the form $J_{x}= \psi(x)e^{ -\rho(x)}$ with $\rho$ some norm and $\psi$ an subexponential correction, we show under appropriate assumptions that given $s\in\mathbb{S}^{d-1}$, the Laplace transform of the two-point function in the direction $s$ is infinite for $\beta=\beta_{\text{sat}}(s)$ (where $\beta_{\text{sat}}(s)$ is a the biggest value such that the inverse correlation length $\nu_{\beta}(s)$ associated to the truncated two-point function is equal to $\rho(s)$ on $[0,\beta_{\text{sat}}(s)))$. Moreover, we prove that the two-point function satisfies Ornstein-Zernike asymptotics for $\beta=\beta_{\text{sat}}(s)$ on $\mathbb{Z}$. As far as we know, this constitutes the first result on the behaviour of the two-point function at $\beta_{\text{sat}}(s)$. Finally, we show that there exists $\beta_{0}$ such that for every $\beta>\beta_{0}$, $\nu_{\beta}(s)=\rho(s)$. All the results are new.