A063610 Smallest k such that 7^k has exactly n 4's in its decimal representation.

1, 2, 10, 15, 26, 32, 33, 58, 62, 50, 46, 89, 102, 108, 90, 118, 130, 122, 146, 144, 112, 138, 196, 224, 226, 212, 256, 250, 259, 239, 218, 254, 386, 260, 318, 292, 353, 321, 358, 326, 392, 401, 330, 396, 427, 442, 438, 443, 450, 449, 474

Offset: 0

Author

Robert G. Wilson v, Aug 10 2001

Keywords

• base
• nonn

Links

• Robert Price, Table of n, a(n) for n = 0..2000

Programs

Mathematica

a = {}; Do[k = 1; While[ Count[ IntegerDigits[7^k], 4] != n, k++ ]; a = Append[a, k], {n, 0, 50} ]; a
Module[{nn=50,p7s},p7s=Table[DigitCount[7^n,10,4],{n,20nn}];Join[{1}, Table[ Position[p7s,i,{1},1],{i,nn}]]]//Flatten (* Harvey P. Dale, Jun 13 2016 *)

Extensions

Name corrected by Jon E. Schoenfield, Jun 26 2018