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A216164
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Period of powers of 7 mod 10^n.
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2
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4, 4, 20, 100, 500, 5000, 50000, 500000, 5000000, 50000000, 500000000, 5000000000, 50000000000, 500000000000, 5000000000000, 50000000000000, 500000000000000, 5000000000000000, 50000000000000000, 500000000000000000
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OFFSET
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1,1
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COMMENTS
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LINKS
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FORMULA
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a(n) = 4*5^(n-2), 1 < n < 5.
a(n) = 5*10^(n-3), n >= 5.
a(n) = 2^(n-3)*5^(n-2) for n>4. a(n) = 10*a(n-1) for n>5. G.f.: 4*x*(125*x^4 +25*x^3 +5*x^2 +9*x -1) / (10*x -1). - Colin Barker, Sep 22 2013
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MATHEMATICA
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Join[{4, 4, 20, 100}, NestList[10*# &, 500, 20]] (* Paolo Xausa, Feb 19 2024 *)
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CROSSREFS
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KEYWORD
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nonn,base,easy
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AUTHOR
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STATUS
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approved
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