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A285933
a(n) = smallest k such that (6*k-3)*2^n-1 and (6*k-3)*2^n+1 are twin primes.
1
1, 1, 2, 3, 14, 1, 2, 10, 8, 3, 17, 28, 62, 8, 58, 20, 64, 1, 12, 75, 14, 6, 197, 41, 128, 63, 14, 65, 8, 58, 114, 98, 63, 45, 124, 36, 72, 516, 28, 45, 43, 183, 2, 25, 142, 68, 249, 30, 324, 155, 188, 200, 334, 56, 87, 178, 98, 110, 22, 25, 24, 70, 2, 271, 17, 498, 412, 750, 877
OFFSET
1,3
COMMENTS
Conjecture: a(n) is ~ (n*log(2))^2/9 as n increases.
EXAMPLE
(6*1-3)*2^1-1 = 5, (6*1-3)*2^1+1 = 7; 5 and 7 are twin primes so a(1) = 1.
(6*1-3)*2^2-1 = 11, (6*1-3)*2^2+1 = 13; 11 and 13 are twin primes so a(2) = 1.
MATHEMATICA
Table[k = 1; While[Times @@ Boole@ PrimeQ[(6 k - 3) 2^n + {-1, 1}] < 1, k++]; k, {n, 69}] (* Michael De Vlieger, May 04 2017 *)
PROG
(PARI) a(n) = {my(k=1); while (!isprime((6*k-3)*2^n-1) || !isprime((6*k-3)*2^n+1), k++); k; } \\ Michel Marcus, May 01 2017
CROSSREFS
Cf. A285808.
Sequence in context: A253575 A027673 A306039 * A281754 A282119 A287096
KEYWORD
nonn
AUTHOR
Pierre CAMI, Apr 29 2017
STATUS
approved