“…Problem (1) has numerous physical applications, e.g., in the field of magnetostatics, where u is the magnetic scalar potential and a is the magnetic permeability; in the field of compressible flow, where u is the velocity potential and a is the density. There have been many numerical results about problems of this kind in bounded domains, for example, the existence and uniqueness of weak solution [8,9], the finite element method [10][11][12], the mixed finite element method [13][14][15], the discontinuous Galerkin finite element method [16][17][18], the weak Galerkin finite element method [19], and the adaptive finite element method [20,21].…”