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 A119132 #12 Mar 31 2023 15:36:38
%S A119132 1,6,15,55,66,561,666,5151,5565,6555,55611,56616,111156,511566,565516,
%T A119132 651511,1565565,5556111,5566116,5656566,5666661,15615666,16666651,
%U A119132 51151555,55561611,61566156,65111166,116151661,555561111,555661116,556561566,556661661
%N A119132 Triangular numbers composed of digits {1,5,6}.
%H A119132 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr156.html">Tridigital Triangular Numbers</a>.
%F A119132 a(n) = A000217(A119133(n)). - _Tyler Busby_, Mar 31 2023
%t A119132 Select[FromDigits/@Flatten[Table[Tuples[{1,5,6},n],{n,9}], 1],OddQ[ Sqrt[ 8#+1]]&] (* _Harvey P. Dale_, Jul 25 2015 *)
%Y A119132 Cf. A000217, A053903, A119133. See A119033 for a table of cross-references.
%K A119132 nonn,base
%O A119132 1,2
%A A119132 _Giovanni Resta_, May 10 2006
%E A119132 a(31)-a(32) from _Tyler Busby_, Mar 23 2023