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 A119148 #11 Apr 06 2023 20:03:38
%S A119148 1,91,171,1711,171991,791911,1717771191,911977119991,999999911791,
%T A119148 1771177919779971,977779999771199191,17979999719799917971,
%U A119148 7197199171171779119991779991
%N A119148 Triangular numbers composed of digits {1,7,9}.
%C A119148 a(14) > 10^40. - _Tyler Busby_, Mar 29 2023
%H A119148 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr179.html">Tridigital Triangular Numbers</a>.
%F A119148 a(n) = A000217(A119149(n)). - _Tyler Busby_, Mar 29 2023
%Y A119148 Cf. A000217, A119149. See A119033 for a table of cross-references.
%K A119148 nonn,base,more
%O A119148 1,2
%A A119148 _Giovanni Resta_, May 10 2006