Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #7 Mar 21 2015 01:36:25
%S 1,1,1,12,1,25,3,13,7,153,43,423,52,916,136,1111,1270,442,2737,975
%N If M(n) is the n-th Mersenne prime, then a(n) is the smallest positive integer such that 2*a(n)*M(n)*M(n+1)-1 is prime.
%C a(21) > 4200. - _Robert G. Wilson v_
%C a(21) > 12000, a(22)=6669, a(23) > 10000, a(24)=5970. - _Ray G. Opao_, Apr 15 2004
%e a(4) = 12: 2(12)(2^7-1)(2^13-1)-1 = 24(127)(8191)-1 = 24966167, which is prime.
%t p = { (* the list of Mersenne exponents in A000043 *) }; f[n_] := Block[{k = 1, q = 2^p[[n]] - 1, r = 2^p[[n + 1]] - 1}, While[ !PrimeQ[2k*q*r - 1], k++ ]; k]; Do[ Print[ f[n]], {n, 25}] (* _Robert G. Wilson v_, Apr 10 2004 *)
%K nonn
%O 1,4
%A _Ray G. Opao_, Apr 08 2004