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 A119113 #20 Mar 31 2023 00:51:00
%S A119113 1,3,15,55,153,351,5151,33153,35511,3531153,3555111,35133153,51151555,
%T A119113 51313515,355311153,355551111,1553111511,5135151153,5351313331,
%U A119113 15533353153,35553111153,35555511111,153335313531,155315331153,315331155153,355333351555,1353153151155
%N A119113 Triangular numbers composed of digits {1,3,5}.
%H A119113 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr135.html">Tridigital Triangular Numbers</a>.
%F A119113 a(n) = A000217(A119114(n)). - _Michel Marcus_, Mar 22 2023
%t A119113 Select[Accumulate[Range[1000000]],Union[Drop[DigitCount[#], {1,5,2}]]=={0}&] (* _Harvey P. Dale_, Apr 16 2011 *)
%t A119113 Table[Select[FromDigits/@Tuples[{1,3,5},n],OddQ[Sqrt[8#+1]]&],{n,12}]//Flatten (* _Harvey P. Dale_, Aug 02 2022 *)
%Y A119113 Cf. A000217, A119114. See A119033 for a table of cross-references.
%K A119113 nonn,base
%O A119113 1,2
%A A119113 _Giovanni Resta_, May 10 2006
%E A119113 a(25)-a(27) from _Tyler Busby_, Mar 22 2023