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 A075597 #11 Nov 11 2022 11:51:34
%S A075597 3,131,11311,3113111,231131111,22311311119,3223113111199,
%T A075597 132231131111993,11322311311119931,7113223113111199313,
%U A075597 171132231131111993137,31711322311311119931373,3317113223113111199313733
%N A075597 a(1) = 3 and then the smallest prime that is obtained by placing digits on both sides of the previous term. Or smallest prime that encompasses a(n-1).
%t A075597 NestList[Select[Flatten[Table[FromDigits[Join[{a},IntegerDigits[#],{b}]],{a,9},{b,1,9,2}]],PrimeQ][[1]]&,3,12] (* _Harvey P. Dale_, Nov 11 2022 *)
%Y A075597 Cf. A075595, A075596, A075598, A075599, A075600, A075601, A075602.
%K A075597 base,nonn
%O A075597 1,1
%A A075597 _Amarnath Murthy_, Sep 28 2002
%E A075597 More terms from _Sascha Kurz_, Jan 20 2003
%E A075597 One more term from _David Wasserman_, Jan 20 2005