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 A101757 #25 Feb 16 2025 08:32:55
%S A101757 5,12,18,42,44,50,58,67,74,83,90,95,126,142,194,210,211,275,367,382,
%T A101757 479,655,734,818,839,926,939,1047
%N A101757 Indices of semiprimes in A000073.
%C A101757 a(29) >= 1091. A000073(1091) is a 288-digit composite number with unknown factorization. Other possible terms are 1390, 1630, 2034, 2131, 2339, ... - _Tyler Busby_, Feb 09 2023
%H A101757 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Semiprime.html">Semiprime</a>
%F A101757 A000073(a(n)) = A101758(n).
%t A101757 Flatten[Position[LinearRecurrence[{1,1,1},{1,1,2},100],_?(PrimeOmega[ #]==2&)]]+1 (* This generates the first 12 terms of the sequence. To generate more, increase 100 to a higher number, but it will take increasingly long to run. *) (* _Harvey P. Dale_, May 31 2013 *)
%Y A101757 Cf. A000073, A001358, A101758.
%Y A101757 Equivalent sequence for primes: A303263.
%K A101757 nonn,more,less
%O A101757 1,1
%A A101757 _Jonathan Vos Post_ and _Ray Chandler_, Dec 26 2004
%E A101757 More terms from _David Wasserman_, Apr 03 2008