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%I #11 Apr 01 2023 09:11:28
%S 1,21,91,2211,222111,22221111,229119121,291212911,2222211111,
%T 222222111111,299992122991,1122921211191,22222221111111,
%U 2222222211111111,21212929921912191,222222222111111111,22222222221111111111,2222222222211111111111,191929912211129991991191
%N Triangular numbers composed of digits {1,2,9}.
%H Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr129.html">Tridigital Triangular Numbers</a>.
%F a(n) = A000217(A119110(n)). - _Tyler Busby_, Mar 31 2023
%Y Cf. A000217, A053889, A119110. See A119033 for a table of cross-references.
%K nonn,base
%O 1,2
%A _Giovanni Resta_, May 10 2006
%E a(17)-a(19) from _Tyler Busby_, Mar 22 2023
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