%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