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Smallest k such that 7^k has exactly n 1's in its decimal representation.
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%I #8 Jan 12 2019 19:15:44

%S 1,4,6,12,19,34,24,37,38,57,52,92,96,88,116,142,133,156,108,147,115,

%T 158,209,190,180,266,200,270,240,262,263,208,279,366,339,304,352,331,

%U 406,316,407,385,396,434,437,376,386,414,470,450,551

%N Smallest k such that 7^k has exactly n 1's in its decimal representation.

%t a = {}; Do[k = 1; While[ Count[ IntegerDigits[7^k], 1] != n, k++ ]; a = Append[a, k], {n, 0, 50} ]; a

%t Table[Position[DigitCount[7^Range[600],10,1],n,1,1],{n,0,50}]//Flatten (* _Harvey P. Dale_, Jan 12 2019 *)

%K base,nonn

%O 0,2

%A _Robert G. Wilson v_, Aug 10 2001

%E Name corrected by _Jon E. Schoenfield_, Jun 26 2018