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 A119121 #13 Mar 28 2023 10:48:13
%S A119121 1,3,91,193131,9939111,119993391339919111,199913191913191191,
%T A119121 399139119131339931,139191313119199339131,1313933311991311991139331,
%U A119121 3131199393919119319199931,31313993393319311993391131311,1191131333139133131313199933931
%N A119121 Triangular numbers composed of digits {1,3,9}.
%H A119121 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr139.html">Tridigital Triangular Numbers</a>.
%F A119121 a(n) = A000217(A119122(n)).
%Y A119121 Cf. A000217, A119122. See A119033 for a table of cross-references.
%K A119121 nonn,base
%O A119121 1,2
%A A119121 _Giovanni Resta_, May 10 2006
%E A119121 a(13) from _Tyler Busby_, Mar 28 2023