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%I #15 Jun 26 2018 19:18:24
%S 1,7,15,34,31,61,42,55,74,82,96,134,110,127,111,160,164,126,141,194,
%T 247,250,223,255,269,281,258,292,378,321,276,340,435,418,407,412,425,
%U 475,365,422,462,476,533,479,495,429,538,526,543,542,625
%N Smallest k such that 5^k has exactly n 7's in its decimal representation.
%t a = {}; Do[k = 1; While[ Count[ IntegerDigits[5^k], 7] != n, k++ ]; a = Append[a, k], {n, 0, 50} ]; a
%t nn=60; With[{c=DigitCount[#,10,7]&/@(5^Range[20nn])}, Flatten[ Table[ Position[c,n,1,1], {n,0,nn}]]] (* _Harvey P. Dale_, Apr 01 2012 *)
%K base,nonn
%O 0,2
%A _Robert G. Wilson v_, Aug 10 2001
%E Name corrected by _Jon E. Schoenfield_, Jun 26 2018