# Mersenne conjectures

In mathematics, the Mersenne conjectures concern the characterization of prime numbers of a form called Mersenne primes, meaning prime numbers that are a power of two minus one.

## Original Mersenne conjecture

The original, called Mersenne's conjecture, was a statement by Marin Mersenne in his Cogitata Physica-Mathematica (1644; see e.g. Dickson 1919) that the numbers $2^n - 1$ were prime for n = 2, 3, 5, 7, 13, 17, 19, 31, 67, 127 and 257, and were composite for all other positive integers n < 257. Due to the size of these numbers, Mersenne did not and could not test all of them, nor could his peers in the 17th century. It was eventually determined, after three centuries and the availability of new techniques such as the Lucas–Lehmer test, that Mersenne's conjecture contained five errors, namely two are composite (n = 67, 257) and three omitted primes (n = 61, 89, 107). The correct list is: n = 2, 3, 5, 7, 13, 17, 19, 31, 61, 89, 107 and 127.

While Mersenne's original conjecture is false, it has led to the New Mersenne conjecture and the Lenstra–Pomerance–Wagstaff conjecture.

## New Mersenne conjecture

The New Mersenne conjecture or Bateman, Selfridge and Wagstaff conjecture (Bateman et al. 1989) states that for any odd natural number p, if any two of the following conditions hold, then so does the third:

1. p = 2k ± 1 or p = 4k ± 3 for some natural number k.
2. 2p − 1 is prime (a Mersenne prime).
3. (2p + 1) / 3 is prime (a Wagstaff prime).

If p is an odd composite number, then 2p − 1 and (2p + 1)/3 are both composite. Therefore it is only necessary to test primes to verify the truth of the conjecture.

Currently, the known numbers for which all three conditions hold are: 3, 5, 7, 13, 17, 19, 31, 61, 127.

Notice the two primes which Mersenne makes fault (67 and 257) are both in the conjecture (67=26+3, 257=28+1), but 89 and 107 are not.

The New Mersenne conjecture can be thought of as an attempt to salvage the centuries-old Mersenne's conjecture, which is false. However, according to Robert D. Silverman, John Selfridge agreed that the New Mersenne conjecture is "obviously true" as it was chosen to fit the known data and counter-examples beyond those cases are exceedingly unlikely. It may be regarded more as a curious observation than as an open question in need of proving.

Renaud Lifchitz has shown that the NMC is true for all integers less than or equal to 16,777,212 by systematically testing all odd primes for which it is already known that one of the conditions holds. His website documents the verification of results up to this number. Another, currently more up-to-date status page on the NMC is The New Mersenne Prime conjecture.

## Lenstra–Pomerance–Wagstaff conjecture

Lenstra, Pomerance, and Wagstaff have conjectured that there is an infinite number of Mersenne primes, and, more precisely, that the number of Mersenne primes less than x is asymptotically approximated by

$e^\gamma\cdot\log \log(x)/\log(2),$[1]

where γ is the Euler–Mascheroni constant. In other words, the number of Mersenne primes with exponent p less than y is asymptotically

$e^\gamma\cdot\log(y)/\log(2).$[1]

This means that there should on average be about $e^\gamma\cdot\log(10)/\log(2)$ ≈ 5.92 primes p of a given number of decimal digits such that $M_p$ is prime.

## References

1. ^ a b Heuristics: Deriving the Wagstaff Mersenne Conjecture. Primes.utm.edu. Retrieved on 2014-05-11.