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 A069602 #15 Feb 08 2025 16:55:51
%S A069602 1,9,9,9,21,9,51,21,9,57,301,51,51,33,209,111,87,153,121,87,63,39,77,
%T A069602 27,57,81,129,147,111,21,147,321,69,93,153,621,817,129,81,803,129,153,
%U A069602 451,171,717,801,959,459,187,291,231,533,399,291,289,869,489,171,381,667,21
%N A069602 a(1) = 1; a(n) = smallest composite number such that the juxtaposition a(1)a(2)...a(n) is a prime.
%H A069602 Harvey P. Dale, <a href="/A069602/b069602.txt">Table of n, a(n) for n = 1..200</a>
%e A069602 a(5) = 21 and the number 199921 is a prime.
%t A069602 a[1] = 1; a[n_] := a[n] = Block[{k = 3, c = IntegerDigits @ Table[ a[i], {i, n - 1}]}, While[PrimeQ[k] || !PrimeQ[FromDigits @ Flatten @ Append[c, IntegerDigits[k]]], k += 2]; k]; Table[ a[n], {n, 61}] (* _Robert G. Wilson v_, Aug 05 2005 *)
%t A069602 nxt[{jx_,a_}]:=Module[{c=9},While[PrimeQ[c]||CompositeQ[jx*10^IntegerLength[c]+c],c+=2];{jx*10^IntegerLength[c]+c,c}]; NestList[nxt,{1,1},60][[;;,2]] (* _Harvey P. Dale_, Feb 08 2025 *)
%Y A069602 Cf. A033680, A074336, A092528.
%K A069602 nonn,base
%O A069602 1,2
%A A069602 _Amarnath Murthy_, Mar 26 2002
%E A069602 More terms from Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Mar 31 2003