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%I #46 Mar 12 2019 09:21:43
%S 6,21,155,889,106483,2228207,9961453,66571993057,
%T 140656423562035331011,55088331748199422233011027879,
%U 17361742620725829882898847100829589,21607930299479592429924287571917281427329
%N a(n) = A000043(n) * A000668(n).
%C Note that a(3)=155 and a(4)=889 also belong to A119691.
%C Old name was: Composite numbers such that the first factor is the relevant exponent of Mersenne prime (the second factor).
%C a(n) is the smallest k > 0 such that A000668(n)^2 divides 2^k-1. Then lpf(2^k-1) = A000668(n), where lpf(m) = A020639(m). - _Thomas Ordowski_, Feb 03 2019
%F a(n) = A000043(n)*A000668(n). - _R. J. Mathar_, Oct 18 2007
%F a(n) = A002326((A000668(n)^2 - 1)/2). - _Thomas Ordowski_, Feb 03 2019
%o (PARI) expm = [2, 3, 5, 7, 13, 17, 19, 31, 61, 89, 107, 127]; vector(#expm, i, expm[i]*(2^expm[i]-1)) \\ where expm comes from A000043; _Michel Marcus_, Feb 07 2019
%Y Cf. A000043, A000668, A002326, A020639, A119691.
%K nonn,less
%O 1,1
%A _A.K. Devaraj_, Aug 02 2007, Aug 06 2007
%E Terms corrected, edited, and new name by _Michel Marcus_, Apr 30 2013
%E a(12) from _Michel Marcus_, Feb 07 2019
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