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%I #7 Jan 21 2015 03:16:41
%S 3,7,31,127
%N Mersenne primes p such that M(p) = 2^p - 1 is also a (Mersenne) prime.
%C Same as exponents of double Mersenne primes. Only four terms are known.
%D R. K. Guy, Unsolved Problems in Number Theory, 3rd ed., Springer-Verlag, NY, 2004, Sec. A3.
%D G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers, 3rd ed., Oxford Univ. Press, 1954, p. 16.
%D P. Ribenboim, The New Book of Prime Number Records, Springer-Verlag, NY, 1996, Chap. 2, Sec. VII.
%H C. K. Caldwell, <a href="http://www.utm.edu/research/primes/mersenne/index.html#unknown">Mersenne Primes: Conjectures and Unsolved Problems</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/DoubleMersenneNumber.html">Double Mersenne Number</a>
%H Wikipedia, <a href="http://www.wikipedia.org/wiki/Mersenne_prime">Mersenne prime</a>
%e 2^2 - 1 = 3 and 2^3 - 1 = 7 are Mersenne primes, so 3 is a member.
%Y Cf. A000043, A000668, A001348, A077586, A103902.
%K nonn,more,hard
%O 1,1
%A _Jonathan Sondow_, Feb 20 2005
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