A119132 Triangular numbers composed of digits {1,5,6}.

1, 6, 15, 55, 66, 561, 666, 5151, 5565, 6555, 55611, 56616, 111156, 511566, 565516, 651511, 1565565, 5556111, 5566116, 5656566, 5666661, 15615666, 16666651, 51151555, 55561611, 61566156, 65111166, 116151661, 555561111, 555661116, 556561566, 556661661

Offset: 1

Author

Giovanni Resta, May 10 2006

Keywords

• nonn
• base

Links

• Giovanni Resta, Tridigital Triangular Numbers.

Crossrefs

Cf. A000217, A053903, A119133. See A119033 for a table of cross-references.

Programs

Mathematica

Select[FromDigits/@Flatten[Table[Tuples[{1,5,6},n],{n,9}], 1],OddQ[ Sqrt[ 8#+1]]&] (* Harvey P. Dale, Jul 25 2015 *)

Formula

a(n) = A000217(A119133(n)). - Tyler Busby, Mar 31 2023

Extensions

a(31)-a(32) from Tyler Busby, Mar 23 2023