Displaying A Fourier Series

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  • #8304
    yderay
    Participant

      BS”D

      Is there any way I would be able to display a fourier series up to some given n without having to write each element of summation up to that point by hand?

      #8337
      david
      Moderator

        Fourier Series are a little bit tricky to do right. There is the DFT that maps a list of N values to N complex values. But some questions what you are looking for here.

        Q: Is the number of data points a power of 2, i.e. 512,1024 etc. If N is large and a prime this is very slow, to the point of being considered an input error.

        Q: Are you interested in the magnitude of the values, often called the spectrum. The N values you get back from the raw DFT really only have N/2 complex values since you hand in real numbers. And as you get closer to the N/2 index the values are inaccurate if you assume that the inital N values were sampling a function that you want the FS for.

        Q: What comes out is technically amplitudes of standing frequencies in the domain. And technically if you have a domain from [2,3] and 100 values that means you have values at 2,2.01,…2.99, i.e. the value at 3 is assumed to be the same as the value at 2 and not repeated in the list. If it was you really should have 513,1025 values etc so that the signal to the DFT has 512,1024 etc. It isn’t that slow to handle 100 values, but large Mersenne prime numbers will be very slow (such as 524287).

         

        Where this would fit in is in a new mechanism similar to the Plot Action column. That column uses a x,y pair of columns and returns a single column. The Fourier Series would take in a single column and return another column. Let’s call this a Column Action for now. The function evaluation would fit naturally into this, and some smoothing options would too. The answer to the above questions would help in fleshing out this functionality.

        #8346
        yderay
        Participant

          BS”D

          Thank you!

          A DFT would definitely be useful for some of the work I’m doing. The kinds of data id be looking at would be osciloscope output and other electrical signals. So it would be some list of amplitudes over time in most cases.

          However, the particular situation I asked this question for is when I’ve already derived the series in hand calculations but want to be able to display it up to some n in the summation without having to write the summation out term by term up to the Nth term. In some ways I guess this is less related to fourier series directly and more related to being able to display a function defined as a summation up to some Nth term.

          #8347
          david
          Moderator

            In the online examples, search for “function with summation”. This shows how to evaluate a power series at a list of x values.

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