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 A069606 #13 Oct 06 2023 18:12:57
%S A069606 4,1,9,11,19,3,3,41,51,51,87,19,63,23,13,29,3,219,183,27,27,3,3,27,
%T A069606 217,129,381,59,163,281,169,57,77,31,9,9,243,147,21,239,39,219,693,37,
%U A069606 143,789,9,163,219,497,51,301,149,103,117,309,591,159,741,131,541,1377,207
%N A069606 a(1) = 4; a(n) = smallest number such that the juxtaposition a(1)a(2)...a(n) is a prime.
%e A069606 a(5) = 19 and the number 4191119 is a prime.
%t A069606 a[1] = 4; a[n_] := a[n] = Block[{k = 1, c = IntegerDigits @ Table[ a[i], {i, n - 1}]}, While[ !PrimeQ[ FromDigits @ Flatten @ Append[c, IntegerDigits[k]]], k += 2]; k]; Table[ a[n], {n, 63}] (* _Robert G. Wilson v_, Aug 05 2005 *)
%t A069606 nxt[{jxt_,a_}]:=Module[{n=1},While[CompositeQ[jxt*10^IntegerLength[n]+n],n++];{jxt*10^IntegerLength[ n]+n,n}]; NestList[nxt,{4,4},70][[;;,2]] (* _Harvey P. Dale_, Oct 06 2023 *)
%Y A069606 Cf. A069602, A069604, A046254, A074340, A092528, A069603, A069605, A069606, A069607, A069608, A069609, A069610, A069611, A111525.
%K A069606 nonn,base
%O A069606 1,1
%A A069606 _Amarnath Murthy_, Mar 26 2002
%E A069606 More terms from _Jason Earls_, Jun 13 2002