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 A129521 #68 Oct 21 2024 04:46:41 %S A129521 6,15,91,703,1891,2701,12403,18721,38503,49141,79003,88831,104653, %T A129521 146611,188191,218791,226801,269011,286903,385003,497503,597871, %U A129521 665281,721801,736291,765703,873181,954271,1056331,1314631,1373653,1537381 %N A129521 Numbers of the form p*q, p and q prime with q=2*p-1. %C A129521 All terms are Fermat 4-pseudoprimes, i.e., satisfy 4^n == 4 (mod n). See A020136 and A122781. %H A129521 David W. Wilson, <a href="/A129521/b129521.txt">Table of n, a(n) for n = 1..1000</a> %H A129521 D. E. Iannucci, <a href="http://math.colgate.edu/~integers/s77/s77.Abstract.html">When the small divisors of a natural number are in arithmetic progression</a>, INTEGERS, Electronic Journal of Combinatorial Number Theory, #77, 2018. See p. 9. %F A129521 a(n) = A005382(n)*A005383(n). %t A129521 p = Select[Prime[Range[155]], PrimeQ[2# - 1] &]; p (2p - 1) (* _Robert G. Wilson v_, Sep 11 2011 *) %o A129521 (Magma) [2*n^2-n: n in [0..1000]|IsPrime(n) and IsPrime(2*n-1)]; // _Vincenzo Librandi_, Dec 27 2010 %o A129521 (PARI) forprime(p=2,10000,q=2*p-1;if(isprime(q),print1(p*q,", "))) %o A129521 (Haskell) %o A129521 a129521 n = p * (2 * p - 1) where p = a005382 n %o A129521 -- _Reinhard Zumkeller_, Nov 10 2013 %Y A129521 Cf. A005382, A005383. %Y A129521 Subsequence of A006881, A129510, and A122781. %Y A129521 Intersection of A000384 and A001358, "hexagonal semiprimes". - _Wesley Ivan Hurt_, Jul 04 2013 %Y A129521 Cf. A141768, A191311, A191592, A245365, A259676, A259677. %K A129521 nonn %O A129521 1,1 %A A129521 _Reinhard Zumkeller_, Apr 19 2007