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%I #88 Apr 14 2020 10:39:34
%S 0,1,5,27,45,47,75,8895,11405,29517,84615,218307
%N Numbers k such that 3^k - k + 1 is prime.
%C 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.
%t 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]
%o (Sage)
%o def list_a(k):
%o return [i for i in range(k) if (3**i) - i + 1 in Primes()]
%o (PARI) isok(k) = isprime(3^k - k + 1); \\ _Jinyuan Wang_, Aug 03 2019
%Y Cf. A100361, A100362.
%K nonn,hard,more
%O 1,3
%A _Giuseppe Bonaccorso_, Aug 02 2019
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