Green-kernel expansion in the Dirichlet space

Fix a bounded planar domain $D$ and explicitly choose the Dirichlet form normalizationIts Dirichlet Green function of the Laplacian satisfies $-{\Delta }_x{G}_D(x,y)=2\pi {\delta }_y(x)$. Let $(e_n)$ be an orthonormal basis of the corresponding Sobolev space $H_0^1(D)$. The formal expansionis interpreted as a weak kernel identity. Concretely, for smooth compactly supported test functions $\varphi ,\psi$,The identity follows by applying Parseval's identity to the two Riesz representatives for the integration functionals. It is not a claim that arbitrary Sobolev space basis functions have pointwise values everywhere or that the series converges on the singular diagonal.

For a finite measure $\rho$ with finite Green energy, the same Hilbert-space identity gives ${\sum }_n(\int e_{n}d\rho {)}^2=\iint G_D(x,y)\rho (dx)\rho (dy)$. This is the square-summability condition for a Finite-Green-energy measure pairing with a Gaussian free field. An alternative energy normalization rescales both the basis and the Green kernel; specifying the factor prevents an unnoticed $2\pi$ mismatch.^[ref]

Let $\rho$ be a finite measure on $D$ with finite Green energy, in the normalization of the Green-kernel expansion in the Dirichlet space. For an orthonormal basis $(e_n)$ and independent standard Gaussian random variables ${\xi }_n$, defineThe sum converges in L2 space, since the sum of the coefficient squares equals the Green energy. It is centered Gaussian withFor several measures, the same coefficients give their cross-covariances by the corresponding mixed Green integrals.

The joint law is independent of the chosen basis. A stronger pathwise statement needs consistent coefficients: realize the field through one isonormal Gaussian process $W$ on the Dirichlet Hilbert space and take ${\xi }_n=W(e_n)$ for each basis. Then Parseval's identity makes both expansions equal to the same $W$-pairing in L2 space. Drawing a fresh independent coefficient family for a different basis produces another field with the same law, not the same outcome-by-outcome field. This distinguishes basis invariance of a construction from an unspecified coupling of two constructions.^[ref]