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A263200
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Number of perfect matchings on a Möbius strip of width 3 and length 2n.
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2
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28, 104, 388, 1448, 5404, 20168, 75268, 280904, 1048348, 3912488, 14601604, 54493928, 203374108, 759002504, 2832635908, 10571541128, 39453528604, 147242573288, 549516764548, 2050824484904, 7653781175068, 28564300215368, 106603419686404, 397849378530248
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OFFSET
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2,1
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COMMENTS
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This sequence obeys the same recurrence relation as A001835.
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LINKS
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FORMULA
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a(n) = Product_{k=1..n} (10 + 2*cos(Pi*(4*k-1)/n) - 12*cos(1/2*Pi*(4*k-1)/n)).
G.f.: 4*x^2*(7-2*x)/(1-4*x+x^2).
a(n) = 2*((2-sqrt(3))^n + (2+sqrt(3))^n).
a(n) = 4*a(n-1) - a(n-2). (End)
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MATHEMATICA
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CoefficientList[Series[4 (7 - 2 x)/(1 - 4 x + x^2), {x, 0, 33}], x] (* Vincenzo Librandi, Oct 12 2015 *)
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PROG
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(PARI) Vec(4*x^2*(7-2*x)/(1-4*x+x^2) + O(x^30)) \\ Altug Alkan, Oct 12 2015
(Magma) I:=[28, 104]; [n le 2 select I[n] else 4*Self(n-1)-Self(n-2): n in [1..30]]; // Vincenzo Librandi, Oct 12 2015
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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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