My problem looks like u/x+u/y=-F(x,y). Boundary conditions are u/x=0|(x=0,x=Lx) and u/y=0|(y=0,y=Ly). This is the Poisson equation. Because the Poisson problem is a special case of the Helmholtz problem. I want to use d_Helmholtz_2D(f, bd_ax, bd_bx, bd_ay, bd_by, bd_az, bd_bz, &xhandle, &yhandle, ipar, dpar, &stat)to solve the eqution with =0. Poisson Library uses the standard five-point finite difference approximation on this mesh to compute the approximation to the solution .It asks for f ,but I have no ideas on setting f on the boundary . In my opinion it is poisson eqution with the Neumann boundary condition,so should I set f=0 in the boundary?Or any advise?Thank you!!