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 A229236 #11 Jun 04 2023 15:09:20
%S A229236 1,3,6,66,3003,66066,617716,3544453,6172882716,30416261403,
%T A229236 61728399382716,66056806460865066,3549632679762369453,
%U A229236 354963215686512369453,30416741529792514761403,35496321045754012369453,3041674104186814014761403
%N A229236 Palindromic triangular number with palindromic index.
%H A229236 Shyam Sunder Gupta, <a href="/A229236/b229236.txt">Table of n, a(n) for n = 1..18</a>
%e A229236 a(5)=3003 because 3003 is 77th triangular number and 77 is palindrome.
%t A229236 Select[Table[{n,(n(n+1))/2},{n,10^8}],AllTrue[#,PalindromeQ]&][[;;,2]] (* _Harvey P. Dale_, Jun 04 2023 *)
%Y A229236 Cf. A003098, A000217, A008509, A008510.
%K A229236 nonn,base
%O A229236 1,2
%A A229236 _Shyam Sunder Gupta_, Sep 17 2013
%E A229236 Previous Mathematica program replaced by _Harvey P. Dale_, Jun 04 2023