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Triangular numbers composed of digits {5,6,7}.
2

%I #15 Jan 23 2025 16:45:36

%S 6,55,66,666,5565,6555,5656566,5676765,6655776,56567566,575675766555,

%T 677767555675,555665666566566,555667666766556,5755656755766655,

%U 565767676566675765,7557657566575767666,5555555666655656666556566566566

%N Triangular numbers composed of digits {5,6,7}.

%C a(19) > 10^40. - _Tyler Busby_, Mar 28 2023

%H Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr567.html">Tridigital Triangular Numbers</a>.

%F a(n) = A000217(A119219(n)). - _Michel Marcus_, Mar 27 2023

%t Select[Flatten[Table[FromDigits/@Tuples[{5,6,7},n],{n,10}]],OddQ[Sqrt[8#+1]]&] (* The program generates the first 10 terms of the sequence. *) (* _Harvey P. Dale_, Jan 23 2025 *)

%Y Cf. A000217, A053969, A119219. See A119033 for a table of cross-references.

%K nonn,base,changed

%O 1,1

%A _Giovanni Resta_, May 10 2006

%E a(18) from _Tyler Busby_, Mar 28 2023