Finite range Decomposition of Gaussian Processes

Let \D be the finite difference Laplacian associated to the lattice \bZ^{d}. For dimension $d\ge 3$, $a\ge 0$ and $L$ a sufficiently large positive dyadic integer, we prove that the integral kernel of the resolvent G^{a}:=(a-\D)^{-1} can be decomposed as an infinite sum of positive semi-definite functions $V_{n}$ of finite range, $V_{n} (x-y) = 0$ for $|x-y|\ge O(L)^{n}$. Equivalently, the Gaussian process on the lattice with covariance $G^{a}$ admits a decomposition into independent Gaussian processes with finite range covariances. For $a=0$, $V_{n}$ has a limiting scaling form $L^{-n(d-2)}\Gamma_{c,\ast}{\bigl (\frac{x-y}{L^{n}}\bigr)}$ as $n\to \infty$. As a corollary, such decompositions also exist for fractional powers (-\D)^{-\alpha/2}, $0<\alpha \leq 2$. The results of this paper give an alternative to the block spin renormalization group on the lattice.Comment: 26 pages, LaTeX, paper in honour of G.Jona-Lasinio.Typos corrected, corrections in section 5 and appendix