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%I #13 Mar 16 2023 04:31:52
%S 1,6,21,66,666,2211,6216,212226,222111,1222266,1261666,16626261,
%T 22121226,22221111,121111266,126222216,661261161,2162616261,
%U 2221211226,2222211111,21166222626,66126661611,166122226621,216262612611,222212111226,222222111111,22162261612611
%N Triangular numbers composed of digits {1,2,6}.
%H Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr126.html">Tridigital Triangular Numbers</a>
%F a(n) = A000217(A119104(n)). - _Michel Marcus_, Mar 16 2023
%t Flatten[Table[Select[FromDigits/@Tuples[{1,2,6},n],OddQ[Sqrt[8#+1]]&],{n,12}]] (* _Harvey P. Dale_, Mar 07 2015 *)
%Y Cf. A000217, A053883, A119104. See A119033 for a table of cross-references.
%K nonn,base
%O 1,2
%A _Giovanni Resta_, May 10 2006
%E a(24)-a(27) from _Tyler Busby_, Mar 14 2023