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A091997
Numbers n such that 3*2^(n-1) - 1 is prime.
2
1, 2, 3, 4, 5, 7, 8, 12, 19, 35, 39, 44, 56, 65, 77, 95, 104, 144, 207, 217, 307, 325, 392, 459, 471, 828, 1275, 3277, 4205, 5135, 7560, 12677, 14899, 18124, 18820, 25691, 26460, 41629, 51388, 71784, 80331, 85688, 88172, 97064, 123631, 155931, 164988, 234761, 414841
OFFSET
1,2
LINKS
Eric Weisstein's World of Mathematics, Thabit ibn Kurrah's rules
FORMULA
a(n) = A002235(n) + 1. - Jinyuan Wang, Jan 30 2020
EXAMPLE
3*2^(3-1) - 1 = 11 so a(1) = 3.
MATHEMATICA
Do[If[PrimeQ[3*2^(n - 1) - 1], Print[n]], {n, 1, 8000}] (* Mohammed Bouayoun (Mohammed.Bouayoun(AT)yahoo.fr), Apr 13 2006 *)
PROG
(PARI) for (i=1, 1000, if(isprime(3*2^(i-1)-1), print1(i, ", ")))
CROSSREFS
Cf. A002235.
Sequence in context: A258085 A247350 A057484 * A124168 A309708 A285929
KEYWORD
nonn
AUTHOR
Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Mar 17 2004
EXTENSIONS
More terms from Mohammed Bouayoun (Mohammed.Bouayoun(AT)yahoo.fr), Apr 13 2006
More terms from Jinyuan Wang, Jan 30 2020
STATUS
approved