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Please, check the following discrete periodic sequence when the period $N=2$.

$x[k]=\exp(j\frac{2\pi}{N}k), N=\text{period}$

$..., x[0]= 1, x[1]= -1, x[2]= 1, x[3]= -1, ... , N=2$

According to my DTFS calculation, DTFS coefficient of above sequence is as follows.

$..., D_{0}=0, D_{1}=1, D_{2}=0, D_{3}=1, ... , N=2 $

If I shift above DTFS by 1 in the frequency domain like this,

$..., D_{0}=1, D_{1}=0, D_{2}=1, D_{3}=0, ...$

Inverse DTFS of above coefficient is as follows according to my calculation.

$..., x[0]=1, x[1]=1, x[2]=1, x[3]=1, ... N=2$

Actually, if DTFS of $x[k]$ is $D_{n}$, then DFTS of $\exp(ja\frac{2\pi}{N}k)x[k]$

will be $D_{n-a}$ by DTFS frequency shifting definition.

So, DFTS of $\exp(j(-1)\frac{2\pi}{N}k)x[k]$ will be $D_{n+1}$.

In this case, $\exp(j(-1)\frac{2\pi}{2}k)x[k]$ is $\exp(-j\frac{2\pi}{2}k)$ $\exp(j\frac{2\pi}{2}k)=1$.

Shifting by $-1$ shows the same result in this case because $N=2$.

So, I think above result is correct.

I know DTFS of $x[k]=1$ with $N=1$ is as follows.

$..., x[0]=1, x[1]=1, x[2]=1, x[3]=1, ... N=1$

My conclusion is as follows.

$x[k]=..., 1, -1, 1, -1,... \longleftrightarrow D_{n}=..., 0, 1, 0, 1,... N=2$ case 1

$x[k]=..., 1, 1, 1, 1,... \longleftrightarrow D_{n}=..., 1, 0, 1, 0,... N=2$ case 2

$x[k]=..., 1, 1, 1, 1,... \longleftrightarrow D_{n}=..., 1, 1, 1, 1,... N=1$ case 3

I can't understand the last 2 lines. I think case 2 and case 3 should have

different sequences in the time domian. What did I miss?

robert bristow-johnson
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    hay kappy, you need to learn how to use $\LaTeX$ math. and you need to define what you mean by "DTFS". – robert bristow-johnson Sep 27 '20 at 17:42
  • now what is the "Discrete Time Fourier Series"? is that the same thing as the "Discrete Fourier Series"? – robert bristow-johnson Sep 30 '20 at 01:28
  • I think most people use the word "DTFS" instead of "DFS". Please, check the chapter 11 and 12 of the following pdf file. http://cfile24.uf.tistory.com/attach/173B49494EC1B160298290

    DTFS, DTFT, Z transform, DFT and FFT are not the same things.

    – kappy super Oct 07 '20 at 15:17
  • Oppenheim and Schafer call the Discrete Fourier Series the "DFS" and it *is* precisely the same thing that the Discrete Fourier Transform (DFT) is but is not the same as the Discrete-Time Fourier Transform (DTFT) nor the Z Transform. "DTFS" is not a term I have ever seen in my 40+ years in electrical engineering and signal processing. – robert bristow-johnson Oct 07 '20 at 16:58
  • If you search the "DTFS" in the ieeexplore.ieee.org, you can find several papers about "DTFS". Please, check the following link.

    https://ieeexplore.ieee.org/search/searchresult.jsp?newsearch=true&queryText=DTFS

    – kappy super Oct 13 '20 at 03:18
  • For properties of DTFS, __1. http://pilot.cnxproject.org/content/collection/col10064/latest/module/m10784/latest

    __2. https://pages.jh.edu/~signals/dtfsproperties2/indexdtfsprops2.htm

    __3. http://web.mit.edu/6.02/www/s2012/handouts/tutprobs/dtfs.html

    __4. http://dsp.cs.cmu.edu/notes/dtfs/dtfs.html

    __5. https://pages.jh.edu/~signals/discretefourier/index.htm

    __6. http://web.cecs.pdx.edu/~ece2xx/ECE223/Slides/FourierPropertiesx4.pdf

    __7.https://web.eecs.umich.edu/~aey/eecs451/lectures/dtfs.pdf

    – kappy super Oct 13 '20 at 05:49
  • One more thing. There is s section about "DTFS" in Oppenheim's book. Please, check the 3.7 (Properties of Discrete-Time Fourier Series) of "Signal & Systems" 2nd Edition (Alan V. Oppenheim, Alan S. Willsky with S. Hamid Nawab). – kappy super Oct 14 '20 at 06:12

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