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

%I #13 Mar 24 2023 08:44:41

%S 6,55,66,666,2556,5565,6555,26565,255255,2265256,5656566,55656525,

%T 525252666,2665266555,2666226566655,6655522556665,56625555222225,

%U 265556626652565,555665666566566,2626662552255225,6225222656552266,26256252552656556,652265255255252222566

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

%H Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr256.html">Tridigital Triangular Numbers</a>

%F a(n) = A000217(A119165(n)).

%t Flatten[Table[Select[FromDigits/@Tuples[{2,5,6},n],OddQ[Sqrt[8#+1]]&],{n,16}]] (* _Harvey P. Dale_, Nov 03 2015 *)

%Y Cf. A000217, A030484, A119165. See A119033 for a table of cross-references.

%K nonn,base

%O 1,1

%A _Giovanni Resta_, May 10 2006

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