Table 3 Relative discrete ℓ²-errors of∇B _z for each iteration number(♯)

$E(\nabla B_z^{{\mathcal{N}}_{\delta }=5})$

$E(\nabla B_z^{{\mathcal{N}}_{\delta }=10})$

$E(\nabla B_z^{{\mathcal{N}}_{\delta }=20})$

$E(\nabla B_z^{{\mathcal{N}}_{\delta }=30})$