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Triangular numbers composed of digits {0,5,6}.
2

%I #11 Mar 22 2023 18:27:15

%S 6,55,66,666,5050,5565,6555,66066,500500,600060,605550,5656566,

%T 50005000,60505500,5000050000,6050055000,500000500000,605000550000,

%U 50000005000000,60500005500000,66605656050060,550006056055665,555665666566566,5000000050000000

%N Triangular numbers composed of digits {0,5,6}.

%H Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr056.html">Tridigital Triangular Numbers</a>

%F a(n) = A000217(A119080(n)). - _Michel Marcus_, Mar 22 2023

%t Rest[Select[FromDigits/@Tuples[{0,5,6},15],IntegerQ[(Sqrt[8#+1]-1)/2]&]] (* _Harvey P. Dale_, Nov 19 2013 *)

%Y Cf. A000217, A058446, A119080. See A119033 for a table of cross-references.

%K nonn,base

%O 1,1

%A _Giovanni Resta_, May 10 2006

%E a(23)-a(24) from _Tyler Busby_, Mar 22 2023