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 A119138 #8 Mar 28 2023 12:40:09
%S A119138 1,15,55,91,595,5151,5995,9591,1511191,51151555,195911115,
%T A119138 511151999595,519191199555,555595555515,595151955555,591915955919511,
%U A119138 15995115959551911,159955559191119511,9199995595111955111955555,1559111919555519991599911911
%N A119138 Triangular numbers composed of digits {1,5,9}.
%H A119138 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr159.html">Tridigital Triangular Numbers</a>.
%F A119138 a(n) = A000217(A119139(n)). - _Tyler Busby_, Mar 28 2023
%Y A119138 Cf. A000217, A119139. See A119033 for a table of cross-references.
%K A119138 nonn,base
%O A119138 1,2
%A A119138 _Giovanni Resta_, May 10 2006
%E A119138 a(20) from _Tyler Busby_, Mar 28 2023