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 A008510 #17 Mar 12 2025 10:38:17
%S A008510 1,2,3,11,77,363,1111,2662,111111,246642,11111111,363474363,
%T A008510 2664444662,26644444662,246644446642,266444444662,2466444446642,
%U A008510 3654345456545434563
%N A008510 Numbers k such that both k and the k-th triangular number are palindromes.
%D A008510 D. Wells, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, 93.
%H A008510 Patrick De Geest, <a href="http://www.worldofnumbers.com/subtria.htm">Subsets of palindromic triangulars</a>
%H A008510 Patrick De Geest, <a href="http://www.worldofnumbers.com/beautiful.htm">For the last term</a>
%H A008510 Shyam Sunder Gupta, <a href="https://doi.org/10.1007/978-981-97-2465-9_3">Triangular Numbers</a>, Exploring the Beauty of Fascinating Numbers, Springer (2025) Ch. 3, 83-125.
%t A008510 palQ[n_] := Reverse[x = IntegerDigits[n]] == x; t = {}; Do[If[palQ[n] && palQ[n*(n + 1)/2], AppendTo[t, n]], {n, 1.2*10^7}]; t (* _Jayanta Basu_, May 15 2013 *)
%K A008510 nonn,base,more
%O A008510 1,2
%A A008510 _Patrick De Geest_