“…(i) truncate an unbounded domain to a bounded one and solve the problem on the bounded domain subject to artificial or transparent boundary conditions [22,26]; (ii) map the original problem on an unbounded domain to one on a bounded domain and use classic spectral methods to solve the new problem [9]; or equivalently, approximate the original problem by some non-classical functions mapped from the classic orthogonal polynomials/functions on a bounded domain [2,3,7,11,12,27,31,34]; (iii) directly approximate the original problem by genuine orthogonal functions such as Laguerre polynomials or functions on the unbounded domain [6,10,13,14,15,16,17,18,19,20,24,30,32,33,35].…”