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 A242846 #31 Mar 27 2021 22:37:00
%S A242846 131,313,1313131313131313131313131,
%T A242846 313131313131313131313131313131313131313131313131313
%N A242846 Palindromic primes of the form ababa...aba containing only the digits 1 and 3.
%C A242846 The next two terms both start with 3 and have 83 and 225 digits, respectively. Those are the only other terms with fewer than 352 digits. Cf. A062216.
%t A242846 Module[{nn=60,a,b},a=Table[FromDigits[Join[PadRight[{},2n,{1,3}],{1}]],{n,nn}];b=Table[FromDigits[Join[PadRight[{},2n,{3,1}],{3}]],{n,nn}];Select[Sort[Join[a,b]],PrimeQ]] (* _Harvey P. Dale_, Sep 07 2020 *)
%Y A242846 Cf. A062216, A032758, A048399, A235154.
%K A242846 nonn,base
%O A242846 1,1
%A A242846 _Felix Fröhlich_, May 23 2014