  We study the behavior of the solutions $u$ of the linear Dirichlet problems
$- \mathrm{div} (M(x) \nabla u) + a(x) u = f(x)$ with respect to perturbations
of the matrix $M(x)$ (with respect to the $G$-convergence) and with respect to
perturbations of the nonnegative coefficient $a(x)$ and of the right hand side
$f(x)$ satisfying the condition $|f (x)| \leq Q \, a (x)$.
