“…Orthogonal polynomials are extensively used for the numerical solution of differential equations in spectral and pseudospectral methods, see for instance, Canuto et al (1988), Doha (1990Doha ( ,1991, Gottlieb and Orszag (1977), Karageorghis (1988a), Phillips (1988,1989) and Lewanowicz (1976Lewanowicz ( ,1986. If these polynomials are used as basis functions, then the rate of decay of the expansion coefficients is determined by the smoothness properties of the function being expanded.…”