A strong mixing condition for second-order stationary random fields

Let ${X_{mn}}$ be a second-order stationary random field on Z². Let ℳ(L) be the linear span of ${X_{mn}: m ≤ 0, n ∈ Z}$, and ℳ(R_N) the linear span of ${X_{mn}: m ≥ N, n ∈ Z}$. Spectral criteria are given for the condition $lim_{N→∞} c_N = 0$, where $c_N$