Double Mersenne number

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

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

where p is a Mersenne prime exponent.

The smallest double Mersenne numbers[edit]

The first four terms of the sequence of double Mersenne numbers are[1] (sequence A077586 in OEIS):

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[edit]

A double Mersenne number that is prime is called a double Mersenne prime. Since a Mersenne number Mp 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 Mp is itself a Mersenne prime. The first values of p for which Mp is prime are p = 2, 3, 5, 7, 13, 17, 19, 31, 61, 89, 107, 127. Of these, M_{M_p} is known to be prime for p = 2, 3, 5, 7. For p = 13, 17, 19, and 31, explicit factors have been found showing that the corresponding double Mersenne numbers are not prime. Thus, the smallest candidate for the next double Mersenne prime is M_{M_{61}}, or 22305843009213693951 − 1. Being approximately 1.695×10694127911065419641, this number is far too large for any currently known primality test. It has no prime factor below 4×1033.[2] There are probably no other double Mersenne primes than the four known.[1][3]

The Catalan–Mersenne number conjecture[edit]

Write M(p) instead of M_p. A special case of the double Mersenne numbers, namely the recursively defined sequence

2, M(2), M(M(2)), M(M(M(2))), M(M(M(M(2)))), ... (sequence A007013 in OEIS)

is called the Catalan–Mersenne numbers.[4] It is said[1] that Catalan came up with this sequence after the discovery of the primality ofM(127) = M(M(M(M(2)))) by Lucas in 1876.[5] Catalan conjectured that they are all prime and that "up to a certain limit," the sequences defined in the same way starting at any Mersenne number are composed only of primes. This limit is now known to be at most 13, because MM13 is not prime.

Although the first five terms (up to M127) are prime, no known methods can decide if any more of these numbers are prime (in any reasonable time) simply because the numbers in question are too huge, unless the primality of MM127 is disproved.

In popular culture[edit]

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".

See also[edit]


  1. ^ a b c Chris Caldwell, Mersenne Primes: History, Theorems and Lists at the Prime Pages.
  2. ^ Tony Forbes, A search for a factor of MM61. Progress: 9 October 2008. This reports a high-water mark of 204204000000×(10019 + 1)×(261 − 1), above 4×1033. Retrieved on 2008-10-22.
  3. ^ I. J. Good. Conjectures concerning the Mersenne numbers. Mathematics of Computation vol. 9 (1955) p. 120-121 [retrieved 2012-10-19]
  4. ^ Weisstein, Eric W., "Catalan-Mersenne Number", MathWorld.
  5. ^ Nouvelle correspondance mathématique vol. 2 (1876), p. 94-96, "Questions proposées" probably collected by the editor. Almost all of the questions are signed by Édouard Lucas as is number 92: "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: "(*) 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, 2p − 1 est un nombre premier p', 2p' − 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.)" [retrieved 2012-10-18]

Further reading[edit]

External links[edit]