Double Mersenne number

{{short description|Number of form 2^(2^p-1)-1 with prime exponent}}

In mathematics, a double Mersenne number is a Mersenne number of the form

: $M_{M_p} = 2^{2^p-1}-1$

where p is prime.

Examples

The first four terms of the sequence of double Mersenne numbers areChris Caldwell, [http://primes.utm.edu/mersenne/index.html#unknown Mersenne Primes: History, Theorems and Lists] at the Prime Pages. {{OEIS|id=A077586}}:

: $M_{M_2} = M_3 = 7$

: $M_{M_3} = M_7 = 127$

: $M_{M_5} = M_{31} = 2147483647$

: $M_{M_7} = M_{127} = 170141183460469231731687303715884105727$

Double Mersenne primes

{{Infobox integer sequence

| name = Double Mersenne primes

| terms_number = 4

| con_number = 4

| first_terms = 7, 127, 2147483647

| largest_known_term = 170141183460469231731687303715884105727

| OEIS = A077586

| OEIS_name = a(n) = 2^(2^prime(n) − 1) − 1

}}

A double Mersenne number that is prime is called a double Mersenne prime. Since a Mersenne number M_p can be prime only if p is prime, (see Mersenne prime for a proof), a double Mersenne number $M_{M_p}$ can be prime only if M_p is itself a Mersenne prime. For the first values of p for which M_p is prime, $M_{M_{p}}$ is known to be prime for p = 2, 3, 5, 7 while explicit factors of $M_{M_{p}}$ have been found for p = 13, 17, 19, and mersenne prime 31.

$p$	$M_{p} = 2^p-1$	$M_{M_{p}} = 2^{2^p-1}-1$	factorization of $M_{M_{p}}$
class="wikitable"
2	3	prime	7
3	7	prime (triple)	127
5	31	prime	2147483647
7	127	prime (quadruple)	170141183460469231731687303715884105727
11	not prime	not prime	47 × 131009 × 178481 × 724639 × 2529391927 × 70676429054711 × 618970019642690137449562111 × ...
13	8191	not prime	338193759479 × 210206826754181103207028761697008013415622289 × ...
17	131071	not prime	231733529 × 64296354767 × ...
19	524287	not prime	62914441 × 5746991873407 × 2106734551102073202633922471 × 824271579602877114508714150039 × 65997004087015989956123720407169 × 4565880376922810768406683467841114102689 × ...
23	not prime	not prime	2351 × 4513 × 13264529 × 285212639 × 76899609737 × ...
29	not prime	not prime	1399 × 2207 × 135607 × 622577 × 16673027617 × 52006801325877583 × 4126110275598714647074087 × ...
31	2147483647	not prime (triple mersenne number)	295257526626031 × 87054709261955177 × 242557615644693265201 × 178021379228511215367151 × ...
37	not prime	not prime
41	not prime	not prime
43	not prime	not prime
47	not prime	not prime
53	not prime	not prime
59	not prime	not prime
61	2305843009213693951	unknown

Thus, the smallest candidate for the next double Mersenne prime is $M_{M_{61}}$ , or 2^{2305843009213693951} − 1.

Being approximately 1.695{{e|694127911065419641}},

this number is far too large for any currently known primality test. It has no prime factor below 1 × 10³⁶.{{cite web |title=Double Mersenne 61 factoring status |url=http://www.doublemersennes.org/mm61.php |website=www.doublemersennes.org |access-date=31 March 2022}}

There are probably no other double Mersenne primes than the four known.[https://www.ams.org/journals/mcom/1955-09-051/S0025-5718-1955-0071444-6/S0025-5718-1955-0071444-6.pdf I. J. Good. Conjectures concerning the Mersenne numbers. Mathematics of Computation vol. 9 (1955) p. 120-121] [retrieved 2012-10-19]

Smallest prime factor of $M_{M_{p}}$ (where p is the nth prime) are

:7, 127, 2147483647, 170141183460469231731687303715884105727, 47, 338193759479, 231733529, 62914441, 2351, 1399, 295257526626031, 18287, 106937, 863, 4703, 138863, 22590223644617, ... (next term is > 1 × 10³⁶) {{OEIS|id=A309130}}

Catalan–Mersenne number conjecture

The recursively defined sequence

: $c_0 = 2$

: $c_{n+1} = 2^{c_n}-1 = M_{c_n}$

is called the sequence of Catalan–Mersenne numbers.{{MathWorld|urlname=Catalan-MersenneNumber|title=Catalan-Mersenne Number}} The first terms of the sequence {{OEIS|id=A007013}} are:

: $c_0 = 2$

: $c_1 = 2^2-1 = 3$

: $c_2 = 2^3-1 = 7$

: $c_3 = 2^7-1 = 127$

: $c_4 = 2^{127}-1 = 170141183460469231731687303715884105727$

: $c_5 = 2^{170141183460469231731687303715884105727}-1 \approx 5.45431 \times 10^{51217599719369681875006054625051616349} \approx 10^{10^{37.70942}}$

Catalan discovered this sequence after the discovery of the primality of $M_{127}=c_4$ by Lucas in 1876.{{cite journal|title=Questions proposées |journal=Nouvelle correspondance mathématique |volume=2 |year=1876 |pages=94–96 |url=https://archive.org/stream/nouvellecorresp01mansgoog#page/n353/mode/2up}} (probably collected by the editor). Almost all of the questions are signed by Édouard Lucas as is number 92: {{quote|Prouver que 2⁶¹ − 1 et 2¹²⁷ − 1 sont des nombres premiers. (É. L.) (*).}} The footnote (indicated by the star) written by the editor Eugène Catalan, is as follows: {{quote|(*) Si l'on admet ces deux propositions, et si l'on observe que 2² − 1, 2³ − 1, 2⁷ − 1 sont aussi des nombres premiers, on a ce théorème empirique: Jusqu'à une certaine limite, si 2ⁿ − 1 est un nombre premier p, 2^p − 1 est un nombre premier p', 2^p' − 1 est un nombre premier p", etc. Cette proposition a quelque analogie avec le théorème suivant, énoncé par Fermat, et dont Euler a montré l'inexactitude: Si n est une puissance de 2, 2ⁿ + 1 est un nombre premier. (E. C.)}}L. E. Dickson, [https://archive.org/details/historyoftheoryo01dick/ History of the theory of numbers. Volume 1: Divisibility and primality] (1919). Published by Washington, Carnegie Institution of Washington.^{p. 22} Catalan conjectured that they are prime "up to a certain limit". Although the first five terms are prime, no known methods can prove that any further terms are prime (in any reasonable time) simply because they are too huge. However, if $c_5$ is not prime, there is a chance to discover this by computing $c_5$ modulo some small prime $p$ (using recursive modular exponentiation). If the resulting residue is zero, $p$ represents a factor of $c_5$ and thus would disprove its primality. Since $c_5$ is a Mersenne number, such a prime factor $p$ would have to be of the form $2kc_4 +1$ . Additionally, because $2^n-1$ is composite when $n$ is composite, the discovery of a composite term in the sequence would preclude the possibility of any further primes in the sequence.

If $c_5$ were prime, it would also contradict the New Mersenne conjecture. It is known that $\frac{2^{c_4} + 1}{3}$ is composite, with factor $886407410000361345663448535540258622490179142922169401 = 5209834514912200c_4 + 1$ .[http://www.hoegge.dk/mersenne/NMC.html#unknown New Mersenne Conjecture]

In popular culture

In the Futurama movie The Beast with a Billion Backs, the double Mersenne number $M_{M_7}$ is briefly seen in "an elementary proof of the Goldbach conjecture". In the movie, this number is known as a "Martian prime".

References

External links

{{MathWorld|urlname=DoubleMersenneNumber|title=Double Mersenne Number}}
Tony Forbes, [http://anthony.d.forbes.googlepages.com/mm61.htm A search for a factor of MM61] {{Webarchive|url=https://web.archive.org/web/20090208194031/http://anthony.d.forbes.googlepages.com/mm61.htm |date=2009-02-08 }}.
[https://web.archive.org/web/20141015012140/http://www.garlic.com/~wedgingt/MMPstats.txt Status of the factorization of double Mersenne numbers]
[http://www.doublemersennes.org Double Mersennes Prime Search]
[http://www.mersenneforum.org/forumdisplay.php?f=99 Operazione Doppi Mersennes]

Category:Eponymous numbers in mathematics

Category:Integer sequences

Category:Large integers

Category:Unsolved problems in number theory

Category:Mersenne primes