Trace estimates and invariance of the essential spectrum
  We provide sufficient conditions under which the difference of the resolvents of two higher-order operators acting in $\R^N$ belongs to trace classes $\cC^p$. We provide explicit estimates on the norm of the resolvent difference in terms of $L^p$ norms of the difference of the coefficients. Such inequalities are useful in estimating the effect of localized perturbations of the coefficients. 
G. Barbatis