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 A133321 #12 Mar 07 2018 18:14:38
%S A133321 15,25,35,55,65,85,95,115,121,143,145,155,185,187,205,215,235,253,265,
%T A133321 295,299,305,335,341,355,365,393,395,411,415,437,445,451,473,485,505,
%U A133321 515,535,545,565,583,635,655,671,679,685,695,717,745,755,781,785,815
%N A133321 Inserting any (identical) digit between adjacent digits of an odd semiprime k never yields a prime.
%C A133321 Odd semiprime analog of A050805. Trivially true for any digit if we substitute "even semiprime" for "odd semiprime." Trivially true for any semiprime which is a multiple of 5 (A001750). The nonmultiples of 5 in this sequence begin 121, 143, 187, 253, 299, 341.
%H A133321 Harvey P. Dale, <a href="/A133321/b133321.txt">Table of n, a(n) for n = 1..1000</a>
%e A133321 121 is in the sequence because 10201, 11211, 12221, 13231, 14241, 15251, 16261, 17271, 18281, 19291 are all composite.
%t A133321 Select[Select[Range[11,900,2],PrimeOmega[#]==2&],AllTrue[Table[ FromDigits[ Riffle[ IntegerDigits[#],n]],{n,0,9}],CompositeQ]&] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Mar 07 2018 *)
%Y A133321 Cf. A001358, A001750, A050674-A050719, A050805, A050806.
%K A133321 base,easy,nonn
%O A133321 1,1
%A A133321 _Jonathan Vos Post_, Oct 18 2007
%E A133321 More terms from _R. J. Mathar_, Oct 22 2007