Componentwise error analysis for FFTs with applications to fast Helmholtz solvers

We analyze the stability of the Cooley-Tukey algorithm for the Fast Fourier Transform of order n = 2(k) and of its inverse by using componentwise error analysis. We prove that the components of the roundoff errors are linearly related to the result in exact arithmetic. We describe the structure of the error matrix and we give optimal bounds for the total error in infinity norm and in L(2),norm. The theoretical upper bounds are based on a ''worst case'' analysis where all the rounding errors work in the same direction. We show by means of a statistical error analysis that in realistic cases the max-norm error grows asymptotically like the logarithm of the sequence length by machine precision. Finally, we use the previous results for introducing tight upper bounds on the algorithmic error for some of the classical fast Helmholtz equation solvers based on the Faster Fourier Transform and for some algorithms used in the study of turbulence.