“…[1,2,6,9,11,12]), considerable progress has been made recently in spectral methods for unbounded domains. Among these methods, a direct and commonly used approach is based on certain orthogonal approximations on infinite intervals, i.e., the Hermite and Laguerre spectral methods ( see, e.g., [5,10,13,16,18,23,24,28,32]). Some authors also developed composite Laguerre-Legendre spectral methods for the half line and mixed Laguerre-Legendre spectral methods for an infinite strip; see [9,15,21,31].…”