A119218 Triangular numbers composed of digits {5,6,7}.

6, 55, 66, 666, 5565, 6555, 5656566, 5676765, 6655776, 56567566, 575675766555, 677767555675, 555665666566566, 555667666766556, 5755656755766655, 565767676566675765, 7557657566575767666, 5555555666655656666556566566566

Offset: 1

Author

Giovanni Resta, May 10 2006

Keywords

• nonn
• base

Comments

a(19) > 10^40. - Tyler Busby, Mar 28 2023

Links

• Giovanni Resta, Tridigital Triangular Numbers.

Crossrefs

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

Programs

Mathematica

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 *)

Formula

a(n) = A000217(A119219(n)). - Michel Marcus, Mar 27 2023

Extensions

a(18) from Tyler Busby, Mar 28 2023