This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A131960 #53 Oct 21 2024 04:36:03 %S A131960 6,21,155,889,106483,2228207,9961453,66571993057, %T A131960 140656423562035331011,55088331748199422233011027879, %U A131960 17361742620725829882898847100829589,21607930299479592429924287571917281427329 %N A131960 a(n) = A000043(n) * A000668(n). %C A131960 Note that a(3) = 155 and a(4) = 889 also belong to A119691. %C A131960 Old name was: Composite numbers such that the first factor is the relevant exponent of Mersenne prime (the second factor). %C A131960 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 %H A131960 Amiram Eldar, <a href="/A131960/b131960.txt">Table of n, a(n) for n = 1..18</a> %F A131960 a(n) = A000043(n)*A000668(n). - _R. J. Mathar_, Oct 18 2007 %F A131960 a(n) = A002326((A000668(n)^2 - 1)/2). - _Thomas Ordowski_, Feb 03 2019 %t A131960 Map[# * (2^#-1) &, MersennePrimeExponent[Range[12]]] (* _Amiram Eldar_, Oct 21 2024 *) %o A131960 (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 A131960 Cf. A000043, A000668, A002326, A020639, A119691. %K A131960 nonn,less %O A131960 1,1 %A A131960 _A.K. Devaraj_, Aug 02 2007, Aug 06 2007 %E A131960 Terms corrected, edited, and new name by _Michel Marcus_, Apr 30 2013 %E A131960 a(12) from _Michel Marcus_, Feb 07 2019