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 A119039 #12 Mar 31 2023 13:21:19
%S A119039 1,10,15,55,105,5050,5151,10011,15051,105111,500500,501501,505515,
%T A119039 510555,10015050,50005000,50015001,50055015,50105055,51010050,
%U A119039 51111105,51151555,110551015,1051501011,1505550501,5000050000,5000150001,5000550015,5001050055,5010055050
%N A119039 Triangular numbers composed of digits {0,1,5}.
%H A119039 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr015.html">Tridigital Triangular Numbers</a>.
%F A119039 a(n) = A000217(A119040(n)). - _Tyler Busby_, Mar 31 2023
%t A119039 Rest[Select[FromDigits/@Tuples[{0, 1, 5}, 12], IntegerQ[(Sqrt[8 # + 1] - 1)/2] &]] (* _Vincenzo Librandi_, Dec 18 2015 *)
%o A119039 (Magma) [t: n in [1..2*10^7] | Set(Intseq(t)) subset {0,1,5} where t is n*(n+1) div 2]; // _Vincenzo Librandi_, Dec 18 2015
%Y A119039 Cf. A000217, A058416, A119040. See A119033 for a table of cross-references.
%K A119039 nonn,base
%O A119039 1,2
%A A119039 _Giovanni Resta_, May 10 2006
%E A119039 More terms from _Vincenzo Librandi_, Dec 18 2015