In the paper [Porkodi and Arumuganathan - Public key cryptosystem based on number theoretic transforms][1] I found the following statement on the second page regarding the Inverse Number Theoretic Transformation (INTT), there the INTT is defined as follows: 

$$h_l = N^{-1} \sum_{k=0}^{N-1} H_k g^{-lk} \pmod{m},\; l=0,\ldots,N-1$$
where $m$ is a composite number and
$$NN^{-1} = 1 \pmod{m},\quad g^N = 1 \pmod{m}$$
and
$$ \sum_{k=0}^{N-1} g^{uk} = 0 \pmod{m}. $$
Equivalently, 
$$ \gcd(g^u - 1, m) = 1 $$
for every $u$ such that $N/u$ is a prime.


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I am now interested in:
1. To know what exactly the condition $\gcd(g^u - 1, m) = 1$ says. For this I have a guess: Assuming $g$ is a primitive root modulo $m$ with order $N$, then $\gcd(g^u - 1, m) = 1$ ensures that $g^u \not\equiv 1 \pmod{m}$, can we say so?
2. How to get the idea that $N/u$ is a prime number and why that is important in this context. I am just interested in how one comes up with this last condition ($\gcd(g^u - 1, m) = 1$
for every $u$ such that $N/u$ is a prime) in the definition.


I look forward to helpful comments.


  [1]: https://publications.waset.org/15309/pdf