  This article shows that the unconditional stability of the Dual-Finite Volume
Method, which is at least valid for linear problems, is not true for generic
nonlinear differential equations including the PMEs unless the coefficient
appearing in the numerical fluxes are appropriately evaluated. This article
provides a theoretically truly isotone numerical fluxes specialized for solving
the PMEs presented, which is still as simple as the conventional fully-upwind
counterpart.
