# How can prime factor of $q-1$ divide prime $q$?

In this paper - "A Subexponential Algorithm for the Discrete Logarithm Problem", author mentions (page 56),

For each $p_l^{e_l} | q$ proceed....

$p_l^{e_l}$ is one of the prime factors of $q-1$, and $q$ is a prime.

How can any number divide prime? Am I missing something here?

Edit1: $q$ is prime, so we can't get any non-trivial factor of $q$. Keeping this in mind, can anyone explain how to proceed with the steps mentioned in the paper. One thing to keep in mind is that $q$ has to be prime to be used in Diffie-Hellman system.

• Presumably it's a typo and $q-1$ is meant, not $q$. – fkraiem Feb 14 '16 at 5:09
• @fkraiem. Can you direct me to any source that says that it's a typo? – Anurag Feb 15 '16 at 11:17

## 2 Answers

For arbitrary positive integer $n$ we have:

$$\gcd(n,n+1)=1$$

So $q-1$ and $q$ haven't any nontrivial common factor.

It probably divides $q-1$ (composite) not $q$.