CS 770 Assignment #5  solution


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1. Implement either the DFT or the FFT algorithm as well as construction of the Fourier approximation of a function from its Fourier coefficients. Make sure to modify the formulas to take into account that Matlab indexing starts at 1. Submit a print-out, as usual
2. Apply your code to the following functions, plot the results of the Fourier approximation and the original function. Choose your own sampling rate n, possibly several ones.
(a) f(x) = x. This function is discontinuous and you will see that a Fourier approximation will oscillate near the discontinuity. This is called the Gibbs phenomenon. (b) f(x) = exp(cos(2πx)). You should expect a good approximation here. (c) f(x) = ((x−0.5)/0.5)2. This function is continuous, but approximation is not as good as in (b). Why? (d) f = ((x − 0.5)/0.5)m, m = 5,10,25,50,.. with a fixed number of sampling points. Explain the observed behavior. It might be easier to plot the error here.
3. (a) Derive by hand the continuous Fourier Series of f(x) = (cos(8πx))4. Compare the discrete and continuous Fourier coefficients with n = 5,11,21. Explain your findings. (b) Repeat with f(x) = x.
4. Derive formulas for using the DFT when f is sampled on an arbitrary interval [a,b]. Modify your code and submit an example of a Fourier approximant on [a,b] for same simple function. 5. (bonus) Come up with a sequence of functions {fk} that are k times differentiable and periodic with period one. Try to see numerically that the error in Fourier approximation decays at about n−k rate.
6. (bonus) Problems 8.1, 8.2 in Moler. File touchtone.zip is posted on Learn.