“…A second order accurate scheme to deal with flux boundary conditions would ruin global fourth order accuracy if a fourth order discretization is utilized in the interior. Research on HOC schemes for flux type boundary conditions can be found, for example, in [4,6,20] for time dependent problems based on operator splitting approaches, for Poisson and Helmholtz or wave equations [1,5,11,16,22,23], other related HOC methods and applications [3,9,10,15,18,20,21,26,27], and for diffusion and advection equations [1,7,14]. However, few can be found in the literature for general elliptic partial differential equations with flux boundary conditions.…”