A275575 Numbers k such that (3^k + 1)/(3 - (-1)^k) is a prime.
2, 3, 4, 5, 7, 13, 16, 23, 32, 43, 64, 281, 359, 487, 577, 1579, 1663, 1741, 3191, 9209, 11257, 12743, 13093, 17027, 26633, 104243, 134227, 152287, 700897, 1205459
Offset: 1
Programs
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Maple
A275575:=n->`if`(isprime((3^n + 1)/(3 - (-1)^n)), n, NULL): seq(A275575(n), n=1..2*10^3); # Wesley Ivan Hurt, Dec 26 2016
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Mathematica
Select[Range[0, 10^3], PrimeQ[(3^# + 1)/(3 - (-1)^#)] &]
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PARI
isok(n) = isprime( (3^n + 1)/(3 - (-1)^n)); \\ Michel Marcus, Dec 26 2016