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%I #24 Dec 24 2020 16:55:11
%S 1,1,1,1,3,1,1,14,5,5,5,1,7,6,5,7,12,1,5,1,6,29,23,20,8,6,6,9,2,10,18,
%T 19,13,57,1,1,9,10,8,5,8,8,26,5,5,6,39,41,6,9,50,6,32,6,4,8,2,79,28,
%U 23,33,78,31,35,19,32,46,7,6,116,39,7,20,6,35,8
%N Least k(n) such that k(n)*3^n*(3^n-1)+j is prime with j= -1 or 1 or both.
%C Sum_{n=1..k} a(n) / Sum_{n=1..k} n tends to 2*log(3)/7.
%H Harvey P. Dale, <a href="/A153090/b153090.txt">Table of n, a(n) for n = 1..1000</a>
%e 1*3^1*(3^1-1)-1=5 prime as 7 so k(1)=1 1*3^2*(3^2-1)-1=71 prime as 73 so k(2)=1
%t lk[n_]:=Module[{c=3^n (3^n-1),k=1},While[NoneTrue[k*c+{1,-1},PrimeQ],k++];k]; Array[lk,90] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Dec 23 2020 *)
%Y Cf. A152414, A152091, A152092, A152093, A152094, A152095.
%K nonn
%O 1,5
%A _Pierre CAMI_, Dec 18 2008
%E Corrected by _Harvey P. Dale_, Dec 23 2020