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 A287218 a(n) = smallest k such that (6*k-3)*2^prime(n) - 1 is prime. 1
 1, 1, 3, 1, 1, 2, 3, 9, 12, 8, 3, 4, 3, 1, 36, 25, 8, 12, 19, 21, 3, 12, 19, 40, 9, 14, 1, 14, 2, 18, 81, 56, 49, 38, 38, 26, 3, 33, 103, 12, 67, 12, 11, 8, 48, 79, 2, 43, 136, 82, 12, 46, 78, 31, 117, 126, 34, 4, 27, 49, 83, 3, 57, 234, 12, 10, 116, 128, 53, 13 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS For n from 1 to 2000, a(n)/prime(n) is always < 1.8. As N increases, (Sum_{n=1..N} a(n)) / (Sum_{n=1..N} prime(n)) tends to log(2)/3; this is consistent with the prime number theorem as the probability that x*2^n-1 is prime with odd x divisible by 3 is ~ 3/(n*log(2)) and after n*log(2)/3 try (n*log(2)/3)*(3/(n*log(2)) = 1. LINKS Pierre CAMI, Table of n, a(n) for n = 1..2000 FORMULA a(n) = A285808(A000040(n)). MATHEMATICA sk[n_]:=Module[{k=1, t=2^Prime[n]}, While[!PrimeQ[(6k-3)*t-1], k++]; k]; Array[ sk, 70] (* Harvey P. Dale, Nov 14 2018 *) CROSSREFS Subsequence of A285808. Cf. A284325, A284631. Sequence in context: A128316 A065836 A078712 * A306438 A221978 A035254 Adjacent sequences:  A287215 A287216 A287217 * A287219 A287220 A287221 KEYWORD nonn AUTHOR Pierre CAMI, May 22 2017 STATUS approved

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Last modified July 11 15:58 EDT 2020. Contains 335626 sequences. (Running on oeis4.)