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A118005
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a(n) = ((-1)^n*5^(n+1) + 9^(n+1))/14.
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1
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1, 4, 61, 424, 4441, 36844, 347221, 3046864, 27812401, 248358484, 2244991981, 20156099704, 181649037961, 1633620638524, 14708689262341, 132347685782944, 1191281759937121, 10720772899980964, 96490770797094301
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OFFSET
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0,2
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COMMENTS
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Number of black cells after n iterations of Haferman's carpet.
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LINKS
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Colin Barker, Table of n, a(n) for n = 0..1000
Eric Weisstein's World of Mathematics, Haferman Carpet
Index entries for linear recurrences with constant coefficients, signature (4,45).
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FORMULA
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From Colin Barker, Jun 08 2013: (Start)
a(n) = 4*a(n-1) + 45*a(n-2).
G.f.: -1 / ((5*x+1)*(9*x-1)).
(End)
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PROG
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(PARI) Vec(1 / ((1 + 5*x)*(1 - 9*x)) + O(x^40)) \\ Colin Barker, Feb 26 2020
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CROSSREFS
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Sequence in context: A158300 A129452 A131014 * A132064 A229666 A252973
Adjacent sequences: A118002 A118003 A118004 * A118006 A118007 A118008
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KEYWORD
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nonn,easy
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AUTHOR
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Eric W. Weisstein, Apr 09 2006
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STATUS
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approved
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