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A053332
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a(n) contains n digits (either '4' or '7') and is divisible by 2^n.
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2
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4, 44, 744, 7744, 47744, 447744, 4447744, 44447744, 444447744, 4444447744, 74444447744, 474444447744, 4474444447744, 44474444447744, 444474444447744, 7444474444447744, 77444474444447744, 477444474444447744
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OFFSET
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1,1
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LINKS
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FORMULA
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a(n) = a(n-1)+10^(n-1)*(4+3*[a(n-1)/2^(n-1) mod 2]) i.e. a(n) ends with a(n-1); if (n-1)-th term is divisible by 2^n then n-th term begins with a 4, if not then n-th term begins with a 7.
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MATHEMATICA
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Flatten[Table[Select[FromDigits/@Tuples[{4, 7}, n], Divisible[#, 2^n]&], {n, 20}]] (* Harvey P. Dale, Jul 25 2011 *)
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CROSSREFS
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KEYWORD
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base,nonn
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AUTHOR
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STATUS
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approved
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