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