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A308807
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a(n) = 4*5^(n-1) + n.
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1
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5, 22, 103, 504, 2505, 12506, 62507, 312508, 1562509, 7812510, 39062511, 195312512, 976562513, 4882812514, 24414062515, 122070312516, 610351562517, 3051757812518, 15258789062519, 76293945312520, 381469726562521, 1907348632812522, 9536743164062523
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OFFSET
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1,1
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COMMENTS
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The last n decimal digits of 2^a(n) form the number 2^n.
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LINKS
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FORMULA
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G.f.: x*(5 - 13*x + 4*x^2) / ((1 - x)^2*(1 - 5*x)).
a(n) = 7*a(n-1) - 11*a(n-2) + 5*a(n-3) for n>3.
(End)
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EXAMPLE
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a(1) = 5, 2^5 = 32, the last digit of 32 is 2, which is 2^1.
a(2) = 22, 2^22 = 4194304, the last 2 digits of 4194304 are 04, which is 2^2.
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MAPLE
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MATHEMATICA
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Table[4*5^(n-1)+n, {n, 30}] (* or *) LinearRecurrence[{7, -11, 5}, {5, 22, 103}, 30] (* Harvey P. Dale, Jun 27 2020 *)
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PROG
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(PARI) Vec(x*(5 - 13*x + 4*x^2) / ((1 - x)^2*(1 - 5*x)) + O(x^25)) \\ Colin Barker, Jun 29 2019
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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