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%I #7 Aug 06 2017 10:28:22
%S 1,1,2,1,2,1,9,4,3,2,17,1,5,7,9,1,2,1,17,11,9,12,20,14,24,6,9,9,41,1,
%T 14,3,2,23,9,3,2,17,59,5,3,2,30,11,21,4,39,21,41,6,32,4,3,2,72,10,39,
%U 9,72,1,36,3,2,14,17,13,84,10,15,4,122,5,6,3,2,10,41,15,5,13,5,13,90,9,38,32
%N Least k such that k*(k+1)*2^n-1 is prime.
%C When k(n)=1 n+1 is prime and the prime is a Mersenne-prime when k(n)=3 then k(n+1)=2 and same prime for n and n+1
%e 1*2*2^1-1=3 prime so k(1)=1
%e 1*2*2^2-1=7 prime so k(2)=1
%e 2*3*2^3-1=47 prime so k(3)=2
%e 9*10*2^7-1=11519 prime so k(7)=9
%t lk[n_]:=Module[{k=1,c=2^n},While[!PrimeQ[k(k+1)c-1],k++];k]; Array[lk,90] (* _Harvey P. Dale_, Aug 06 2017 *)
%K nonn
%O 1,3
%A _Pierre CAMI_, Mar 31 2005
%E Reformulated the definition - _R. J. Mathar_, Nov 13 2009
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