%I #18 Mar 27 2023 03:36:32
%S 990,90779090007770790,79909779970709079970099779979097777700
%N Triangular numbers composed of digits {0,7,9}.
%C a(4) > 10^40.
%H Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr079.html">Tridigital Triangular Numbers</a>.
%F a(n) = A000217(A218402(n)).
%Y Cf. A000217, A218402. See A119033 for a table of cross-references.
%K base,nonn,more,hard,bref
%O 1,1
%A _Max Alekseyev_, Oct 28 2012