“…Along the analytical front, one can refer to [ 3 , 6 ] and the references therein. Along the numerical front, different efficient and accurate numerical methods including finite difference (FD) methods [ 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ], finite element (FE) methods [ 18 , 19 , 20 , 21 , 22 ], spectral method [ 23 ], discontinuous Galerkin method [ 24 ], virtual element method [ 25 ], and so on. Furthermore, there have also been some other efficient algorithms, such as the multigrid methods [ 26 , 27 , 28 ] and two-grid methods [ 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 ], where the idea is presented firstly by Xu in [ 38 ].…”