Divergence-Free: A Crucial Strategy to Speed Up the Convergence of a Multigrid Solver for 3-D Natural Source Electromagnetic Modeling. - Rongwen Guo; Min Yu; Yongfei Wang; Jianxin Liu; Akande Akintunde Abiodun; Dengkang Wang; Xinhao Chen (IEEE Trans. Geosci. Remote. Sens., 2024)