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This question comes right out of the problems section of Oppenheim's Signals & Systems 2ed textbook. I don't understand the solution to the problem.

(problem #1.35) If a discrete-time complex sinusoid is expressed by \$x[n] = \exp(jmn2\pi/N)\$ (m is any integer), prove that the fundamental frequency is N0 = N/gcd(N,m).

Solution: for a discrete-time signal to be periodic \$x[n]= x[n+N_0]\$. Substituting n+N0 into n and expanding that expression means that \$m(2\pi/N)N_0 = 2\pi{}k\$, where k is some (not any) integer. Rearranging we get the expression N0=N/(m/k). somehow (m/k) = gcd(N,m).

I don't see it.

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  • \$\begingroup\$ I added some math formatting to make it easier to read your question. But I also rearranged the first formula, putting n inside the exponential function, because I think this is the correct form for a complex exponential. Please revert if I changed the meaning of your question. \$\endgroup\$
    – The Photon
    Apr 5 '13 at 2:21
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Let's write the book's solution out in more detail.

First, by definition, a discrete-time signal is periodic with period N0 if for all n, \$x[n+N_0] = x[n]\$.

So we're interested in cases where

\$\exp(j2\pi{}mn/N) = \exp(j2\pi{}m(n+N_0)/N)\$.

We know that the complex exponential, considered as a continuous function, repeats whenever it's argument advances by \$2\pi\$. So our equality holds whenever the phase difference between the arguments increases by some multiple of \$2\pi\$, that is whenever there is a k such that

\$2\pi{}mn/N + 2\pi{}k = 2\pi{}m(n+N_0)/N\$.

We can eliminate terms here to get

\$ mn/N + k = mn/N + mN_0/N\$

and further

\$ k = mN_0/N \$

So the period of our discrete time function is

\$N_0 = Nk/m\$

But N0 must be an integer, and we also want the smallest possible value of N0 to get the fundamental frequency. That means we want the smallest k that makes Nk/m an integer.

Lets write this out in terms of the prime factors of N and m:

\$N_0 = \dfrac{k \Pi_p n_p}{\Pi_q m_q} \$

For this to be an integer we need k to be made up of all the factors of m, except the ones that are held in common with N.

\$ k = \dfrac{\Pi_q m_q}{\Pi_{\mathrm{common\ factors}} m_q}\$

But the numerator here is just m, and the denominator here is just the gcd of N and m, so we have

\$ m / k = \gcd(N, m)\$

which is what we were trying to show.

TL;DR: It's to make it work out so that N0 is an integer.

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