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 A104623 #9 Feb 16 2025 08:32:57 %S A104623 4,8,9,11,12,14,15,16,22,23,32,34,37,41,42,50,52,57,58,66,69,76,77,81, %T A104623 90,120,139 %N A104623 Indices of semiprime (A001358) values of Heptanacci-Lucas numbers A104621. %C A104623 The 7th-order linear recurrence A104622 (heptanacci-Lucas numbers) is a generalization of the Lucas sequence A000032. T. D. Noe and I have noted that the heptanacci-Lucas numbers have many more primes than the corresponding heptanacci (see A104414) which he found has only the first 3 primes that I identified through the first 5000 values, whereas these heptanacci-Lucas numbers have 17 primes among the first 100 values. For primes in Heptanacci-Lucas numbers, see A104622. %H A104623 Mario Catalani, <a href="http://arxiv.org/abs/math.CO/0210201">Polymatrix and Generalized Polynacci Numbers</a>, arXiv:math.CO/0210201 v1, 2002 %H A104623 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Semiprime.html">Semiprime</a>. %e A104623 A104621(4) = 15 = 3 * 5, %e A104623 A104621(8) = 247 = 13 * 19, %e A104623 A104621(9) = 493 = 17 * 29, %e A104623 A104621(11) = 1959 = 3 * 653, %e A104623 A104621(12) = 3903 = 3 * 1301, %e A104623 A104621(14) = 15487 = 17 * 911, %Y A104623 Cf. A001358. %K A104623 nonn %O A104623 0,1 %A A104623 _Jonathan Vos Post_, Mar 17 2005