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 A308829 Numbers k such that 3^k - k + 1 is prime. 0
 0, 1, 5, 27, 45, 47, 75, 8895, 11405, 29517, 84615, 218307 (list; graph; refs; listen; history; text; internal format)
 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 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

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Last modified October 22 10:24 EDT 2019. Contains 328317 sequences. (Running on oeis4.)