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 A103902 Mersenne primes p such that the Mersenne number M(p) = 2^p - 1 is composite. 2
 8191, 131071, 524287, 2147483647 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Only four terms are known. The first four Mersenne primes (p=2^q-1 in A000668) are double Mersenne primes, i.e., in A103901. The next four yield a composite M(p) and therefore are in this sequence. The next larger Mersenne prime p = A000668(9) has already 19 digits and is much too large to enable us, as of today, to test the primality of 2^p-1 (which would require over 10^8 gigabytes just to be stored in binary). This explains that only 4 terms are known of this sequence and of A103901; for all the 30+ remaining members of A000668 it is not known whether they belong to A103901 or to this sequence A103902. - M. F. Hasler, Jan 21 2015 REFERENCES R. K. Guy, Unsolved Problems in Number Theory, 3rd ed., Springer-Verlag, NY, 2004, Sec. A3. G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers, 3rd ed., Oxford Univ. Press, 1954, p. 16. P. Ribenboim, The New Book of Prime Number Records, Springer-Verlag, NY, 1996, Chap. 2, Sec. VII. LINKS C. K. Caldwell, Mersenne Primes: Conjectures and Unsolved Problems Eric Weisstein's World of Mathematics, Double Mersenne Number Wikipedia, Mersenne prime EXAMPLE M(13) = 8191 is a Mersenne prime and M(1891) is composite, so 1891 is a member. PROG (PARI) is(n)=isprime(2^n-1) && !isprime(2^(2^n-1)-1) \\ Charles R Greathouse IV, Jan 24 2015 CROSSREFS Cf. A000043, A000668, A001348, A077585, A077586, A103901. Sequence in context: A051334 A145592 A172315 * A075960 A305758 A011563 Adjacent sequences:  A103899 A103900 A103901 * A103903 A103904 A103905 KEYWORD hard,nonn AUTHOR Jonathan Sondow, Feb 20 2005 STATUS approved

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Last modified May 18 12:26 EDT 2021. Contains 343995 sequences. (Running on oeis4.)