Convergence of Spectral Expansions Related to Elliptic Operators with Singular Coefficients

Let $\Omega$ be a smooth domain in $\mathbb{R}^n$ (not necessarily bounded), and let $A$ be a linear elliptic differential operator of order $2m$ with singular coefficients acting in $L^2(\Omega)$. Under some assumptions of singularity for the coefficients of $A$, we consider the Friedrichs extension and study the convergence of spectral expansions in Sobolev spaces.