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 A133980 #7 Apr 28 2015 15:02:09
%S A133980 23,37,211,223,229,241,271,283,311,313,317,331,337,347,353,359,367,
%T A133980 373,379,383,389,397,523,541,547,571,719,743,761,773,797,1117,1123,
%U A133980 1129,1153,1171,1319,1361,1367,1373,1723,1741,1747,1753,1759,1783,1789,1931
%N A133980 Home primes of semiprimes (A001358).
%C A133980 Not the same as A133957.
%C A133980 Number of terms < 10^n: 0, 2, 31, 223, 1638, 11752, 89918, ....
%t A133980 lst = {}; semiPrimeQ[x_] := Plus @@ Last /@ FactorInteger@x == 2; f[n_] := FromDigits@ Flatten[IntegerDigits@ Table[ #[[1]], {#[[2]]}] & /@ FactorInteger@n, 2]; h[n_] := NestWhileList[ f@# &, n, ! PrimeQ@# &, 1, 6];
%Y A133980 Cf. A001358, A133955, A133957.
%K A133980 nonn,base
%O A133980 1,1
%A A133980 _Jonathan Vos Post_ & _Robert G. Wilson v_, Oct 01 2007, Oct 06 2007