Chebyshev domain truncation is inferior to fourier domain truncation for solving problems on an infinite interval

"Domain truncation" is the simple strategy of solving problems on y E [ o o , oo] by using a large but finite computational interval, [ L , L ] . Since u(y) is not a periodic function, spectral methods have usually employed a basis of Chebyshev polynomials, Tn(y/L). In this note, we show that because u(_+ L) must be very, very small if domain truncation is to succeed, it is always more efficient to apply a Fourier expansion instead. Roughly speaking, it requires about 100 Chebyshev polynomials to achieve the same accuracy as 64 Fourier terms. The Fourier expansion of a rapidly decaying but nonperiodic function on a large interval is also a dramatic illustration of the care that is necessary in applying asymptotic coefficient analysis. The behavior of the Fourier coefficients in the limit n --* ~ for fixed interval L is never relevant or significant in this application.