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 A079654 #14 Mar 31 2023 14:12:54
%S A079654 1,171,741,1711,117144471,417417171,7417744447141,
%T A079654 177141144447177414711441,1417711714147441711474771771,
%U A079654 144114474147744777714441111111771711,111411774474177744717747747477177774111
%N A079654 Triangular numbers using only the straight digits 1, 4 and 7.
%C A079654 Probably finite.
%C A079654 a(12) > 10^40 if it exists. - _Tyler Busby_, Mar 22 2023
%H A079654 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr147.html">Tridigital Triangular Numbers</a>.
%F A079654 a(n) = A000217(A119127(n)). - _Tyler Busby_, Mar 31 2023
%t A079654 Do[ If[ Union[ Join[ IntegerDigits[n(n + 1)/2], {1, 4, 7}]] == {1, 4, 7}, Print[n(n + 1)/2]], {n, 0, 3*10^7}]
%Y A079654 Cf. A000217, A028373, A053899, A119127. See A119033 for a table of cross-references.
%K A079654 base,nonn
%O A079654 1,2
%A A079654 _Shyam Sunder Gupta_, Jan 23 2003
%E A079654 Edited and extended by _Robert G. Wilson v_, Jan 24 2003
%E A079654 More terms from _Giovanni Resta_, May 10 2006
%E A079654 a(10)-a(11) from _Tyler Busby_, Mar 22 2023