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%I #8 Jun 26 2018 19:11:21
%S 1,2,10,15,26,32,33,58,62,50,46,89,102,108,90,118,130,122,146,144,112,
%T 138,196,224,226,212,256,250,259,239,218,254,386,260,318,292,353,321,
%U 358,326,392,401,330,396,427,442,438,443,450,449,474
%N Smallest k such that 7^k has exactly n 4's in its decimal representation.
%t a = {}; Do[k = 1; While[ Count[ IntegerDigits[7^k], 4] != n, k++ ]; a = Append[a, k], {n, 0, 50} ]; a
%t 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 *)
%K base,nonn
%O 0,2
%A _Robert G. Wilson v_, Aug 10 2001
%E Name corrected by _Jon E. Schoenfield_, Jun 26 2018
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