Example of an element D^i,j,k,l (dashed lines) using n_a = n_c = n_d = 3 and n_b = 2 points. The left and right panels correspond to 2D slices in the (r, Θ) and (μ, φ) planes, respectively (we display here the local coordinate system given by Eq. B.2). The black dots correspond to the nodes where the solution is computed while the crosses correspond to the interpolated value of I used to compute the numerical flux $F_h^{\ast }$ at the interface, along the Θ coordinate.