A295639 Smallest k not divisible by 3 such that k*3^n + 1 is prime.
2, 2, 4, 2, 2, 2, 8, 8, 2, 8, 28, 10, 64, 4, 4, 2, 2, 10, 20, 26, 56, 8, 104, 16, 34, 14, 14, 20, 26, 2, 26, 26, 14, 22, 26, 16, 22, 50, 4, 62, 64, 68, 88, 70, 56, 34, 146, 32, 50, 20, 314, 8, 40, 2, 70, 22, 2, 8, 40, 2, 64, 14, 136, 100, 2
Offset: 1
Keywords
Links
- Pierre CAMI, Table of n, a(n) for n = 1..4000
Programs
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Maple
f:= proc(n) local i,j,k,t; t:= 3^n; for i from 0 do for j in [2,4] do if isprime((6*i+j)*t+1) then return 6*i+j fi od od end proc: map(f, [$1..100]); # Robert Israel, Dec 14 2017 -
Mathematica
f[n_] := Block[{k = 2}, While[If[Mod[k, 3] == 0, k+=2]; ! PrimeQ[k*3^n + 1], k+=2]; k]; Array[f, 65] (* Robert G. Wilson v, Dec 12 2017 *) -
PARI
a(n) = {k = 1; while (!isprime(k*3^n+1), k++; if (! (k%3), k++)); k;} \\ Michel Marcus, Nov 25 2017
Comments