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 A119170 #21 Apr 06 2023 21:41:42
%S A119170 55,595,5995,255255,292995,299925,25952229225,99292929555225,
%T A119170 55955525255559595,555959229929999252295,925952955522559552259925,
%U A119170 5299552922255552929595999295,5995959559225555222929295255522595295,9925559955529295222599225259529552225
%N A119170 Triangular numbers composed of digits {2,5,9}.
%C A119170 a(15) > 10^40. - _Tyler Busby_, Mar 24 2023
%H A119170 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr259.html">Tridigital Triangular Numbers</a>.
%F A119170 a(n) = A000217(A119171(n)).
%F A119170 a(n) == 5 (mod 10) for all n. - _Chai Wah Wu_, Nov 30 2018
%t A119170 Flatten[Table[Select[FromDigits /@ Tuples[{2, 5, 9}, n], IntegerQ[(Sqrt[8 # + 1] - 1)/2] &], {n, 11}]] (* _Amiram Eldar_, Nov 30 2018 *)
%Y A119170 Cf. A000217, A053929, A119171. See A119033 for a table of cross-references.
%K A119170 nonn,base,more
%O A119170 1,1
%A A119170 _Giovanni Resta_, May 10 2006
%E A119170 a(13)-a(14) from _Tyler Busby_, Mar 24 2023