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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 (list; graph; refs; listen; history; text; internal format)
 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 Adjacent sequences:  A091994 A091995 A091996 * A091998 A091999 A092000 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

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Last modified June 25 04:15 EDT 2021. Contains 345452 sequences. (Running on oeis4.)