  Generalized spectra of differential operators can be related to spectra of
preconditioned discretized operators. Obtaining (estimates of) the eigenvalues
of the preconditioned discretized operators may lead to better estimating of
the quality of preconditioners. In this short paper, we answer the open
question posted in the recent paper Generalized spectrum of second order
differential operators, authored by Gergelits, Nielsen, and Strako\v s. The
proof we present allows us to fully extend characterizing the generalized
spectra of $\nabla\cdot K\nabla u=\lambda\triangle u$ to problems of dimension
three or higher.
