login
A063552
Smallest k such that 2^k has exactly n 7's in its decimal representation.
9
1, 15, 27, 24, 46, 75, 116, 152, 157, 170, 181, 237, 297, 282, 360, 214, 317, 380, 475, 311, 417, 440, 424, 538, 535, 427, 474, 632, 654, 651, 810, 766, 832, 626, 848, 824, 780, 931, 897, 992, 889, 1004, 981, 1079, 1087, 1123, 1224, 1186, 892, 1174, 1200
OFFSET
0,2
LINKS
MATHEMATICA
a = {}; Do[k = 1; While[ Count[ IntegerDigits[2^k], 7] != n, k++ ]; a = Append[a, k], {n, 0, 50} ]; a
Join[{1}, With[{c=DigitCount[#, 10, 7]&/@(2^Range[0, 1300])}, Flatten[ Table[ Position[c, n, 1, 1], {n, 60}]]]-1] (* Harvey P. Dale, Oct 14 2012 *)
PROG
(PARI) a(n)={my(k=1); while(n<>#select(d->d==7, digits(2^k)), k++); k} \\ Harry J. Smith, Aug 26 2009, Andrew Howroyd, Jun 26 2018
KEYWORD
base,nonn
AUTHOR
Robert G. Wilson v, Aug 10 2001
EXTENSIONS
Name corrected by Jon E. Schoenfield, Jun 25 2018
STATUS
approved