Factor of Mersenne Number Mp is of form 2kp + 1
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Theorem
Let $p$ and $q$ be prime numbers such that $q$ is a divisor of the Mersenne number $M_p$.
Then:
- $q = 2 k p + 1$
for some integer $k$.
Proof
Let $q \divides M_p$.
Then:
- $2^p \equiv 1 \pmod q$
From Integer to Power of Multiple of Order, the multiplicative order of $2 \pmod q$ divides $p$.
By Fermat's Little Theorem, the multiplicative order of $2 \pmod q$ also divides $q - 1$.
Hence:
- $q - 1 = 2 k p$
$\blacksquare$
Historical Note
This result was known to Pierre de Fermat by $1640$, and it has been suggested that Marin Mersenne made use of it when providing his famous list of Mersenne primes in his Cogitata Physico-Mathematica of $1644$.
Sources
- 1926: Sir Thomas L. Heath: Euclid: The Thirteen Books of The Elements: Volume 2 (2nd ed.) ... (previous) ... (next): Book $\text{IX}$. Proposition $36$: Footnote