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 A104876 #18 Feb 16 2025 08:32:57
%S A104876 209,510509,6469693229,200560490129,13082761331670029,
%T A104876 1922760350154212639069,557940830126698960967415389,
%U A104876 40729680599249024150621323469,2305567963945518424753102147331756069,232862364358497360900063316880507363069
%N A104876 Semiprimes of the form primorial(k) - 1.
%H A104876 Sebastian Martin Ruiz, <a href="https://www.jstor.org/stable/3619207">A Result on Prime Numbers</a>, Math. Gaz. 81, 269, 1997.
%H A104876 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Primorial.html">Primorial.</a>
%H A104876 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Semiprime.html">Semiprime.</a>
%F A104876 n# - 1 iff semiprime. Equals {A002110(i) - 1} intersection {A001358(j)}.
%e A104876 4# - 1 = 209 = 11 * 19.
%e A104876 7# - 1 = 510509 = 61 * 8369.
%e A104876 10# - 1 = 6469693229 = 79 * 81894851.
%t A104876 Bigomega[n_]:=Plus@@Last/@FactorInteger[n]; SemiprimeQ[n_]:=Bigomega[n]==2; Primorial[n_]:=Product[Prime[i], {i, n}]; Select[Table[Primorial[n]-1, {n, 30}], SemiprimeQ] (* _Ray Chandler_, Mar 28 2005 *)
%Y A104876 Cf. A001358, A002110, A034386, A005234, A014545, A018239, A006794, A057704, A057705, A104877.
%K A104876 easy,nonn
%O A104876 1,1
%A A104876 _Jonathan Vos Post_, Mar 28 2005
%E A104876 Entry revised by _N. J. A. Sloane_, Apr 01 2006