OFFSET
0,2
LINKS
Harry J. Smith, Table of n, a(n) for n = 0..150
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
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Robert G. Wilson v, Aug 10 2001
EXTENSIONS
Name corrected by Jon E. Schoenfield, Jun 25 2018
STATUS
approved