A119105 Triangular numbers composed of digits {1,2,7}.

1, 21, 171, 1711, 2211, 72771, 121771, 222111, 712221, 22221111, 72222171, 2222211111, 12177127711, 72177771711, 171277217121, 222222111111, 22222221111111, 121727717277211, 2222222211111111, 22117121117271271, 222222222111111111, 12172172111271712221

Offset: 1

Author

Giovanni Resta, May 10 2006

Keywords

• nonn
• base

Links

• Giovanni Resta, Tridigital Triangular Numbers.

Crossrefs

Cf. A000217, A053885, A119106. See A119033 for a table of cross-references.

Programs

Mathematica

Flatten[Table[Select[FromDigits/@Tuples[{1,2,7},n],OddQ[Sqrt[8#+1]]&],{n,17}]] (* Harvey P. Dale, Sep 02 2015 *)

Formula

a(n) = A000217(A119106(n)). - Tyler Busby, Mar 31 2023

Extensions

a(21)-a(22) from Tyler Busby, Mar 23 2023