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Triangular numbers composed of digits {1,7,9}.
2

%I #11 Apr 06 2023 20:03:38

%S 1,91,171,1711,171991,791911,1717771191,911977119991,999999911791,

%T 1771177919779971,977779999771199191,17979999719799917971,

%U 7197199171171779119991779991

%N Triangular numbers composed of digits {1,7,9}.

%C a(14) > 10^40. - _Tyler Busby_, Mar 29 2023

%H Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr179.html">Tridigital Triangular Numbers</a>.

%F a(n) = A000217(A119149(n)). - _Tyler Busby_, Mar 29 2023

%Y Cf. A000217, A119149. See A119033 for a table of cross-references.

%K nonn,base,more

%O 1,2

%A _Giovanni Resta_, May 10 2006