A308829 Numbers k such that 3^k - k + 1 is prime.

0, 1, 5, 27, 45, 47, 75, 8895, 11405, 29517, 84615, 218307

Offset

1,3

Comments

Sieving can be limited to odd values of k, because 3^k - k + 1 is even when k is even. In fact, if k is even, 3^k - k is odd and the successor is even.

Links

Table of n, a(n) for n=1..12.

Mathematica

ListA[k_] := Block[{seq = {}, n = 0, i = 0}, While[Length[seq] < k, {n = 3^i - i + 1, If[PrimeQ[n], AppendTo[seq, i]], i += 1}]; seq]

Prog

(Sage)
def list_a(k):
  return [i for i in range(k) if (3**i) - i + 1 in Primes()]
(PARI) isok(k) = isprime(3^k - k + 1); \\ Jinyuan Wang, Aug 03 2019

Crossrefs

Cf. A100361, A100362.

Sequence in context: A156215 A058490 A299578 * A136917 A181890 A048712

Adjacent sequences: A308826 A308827 A308828 * A308830 A308831 A308832

Keyword

nonn,hard,more

Author

Giuseppe Bonaccorso, Aug 02 2019

Status

approved