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A003757
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Number of perfect matchings (or domino tilings) in D_4 X P_(n-1).
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8
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0, 1, 1, 6, 13, 49, 132, 433, 1261, 3942, 11809, 36289, 109824, 335425, 1018849, 3104934, 9443629, 28756657, 87504516, 266383153, 810723277, 2467770054, 7510988353, 22861948801, 69584925696, 211799836801, 644660351425
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OFFSET
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0,4
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COMMENTS
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Here D_4 is the graph on 4 vertices with edges (1,2), (1,3), (2,3), (1.4): a triangular kite with a tail.
This is a divisibility sequence; that is, if n divides m, then a(n) divides a(m). - T. D. Noe, Dec 22 2008
This is the case P1 = 1, P2 = -8, Q = 1 of the 3 parameter family of 4th-order linear divisibility sequences found by Williams and Guy. - Peter Bala, Mar 31 2014
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REFERENCES
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F. Faase, On the number of specific spanning subgraphs of the graphs G X P_n, Ars Combin. 49 (1998), 129-154.
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LINKS
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FORMULA
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a(n) = a(n-1) + 6a(n-2) + a(n-3) - a(n-4), n>4.
G.f.: x(1-x^2)/(1-x-6x^2-x^3+x^4). [T. D. Noe, Dec 22 2008]
a(n) = ( T(n,alpha) - T(n,beta) )/(alpha - beta), where alpha = (1 + sqrt(33))/4 and beta = (1 - sqrt(33))/4 and T(n,x) denotes the Chebyshev polynomial of the first kind.
a(n) = the bottom left entry of the 2 X 2 matrix T(n, M), where M is the 2 X 2 matrix [0, 2; 1, 1/2].
a(n) = U(n-1,i*(1 + sqrt(3))/sqrt(8))*U(n-1,i*(1 - sqrt(3))/sqrt(8)), where U(n,x) denotes the Chebyshev polynomial of the second kind.
See the remarks in A100047 for the general connection between Chebyshev polynomials and 4th-order linear divisibility sequences. (End)
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MATHEMATICA
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CoefficientList[Series[x(1-x^2)/(1-x-6x^2-x^3+x^4), {x, 0, 30}], x] (* T. D. Noe, Dec 22 2008 *)
LinearRecurrence[{1, 6, 1, -1}, {0, 1, 1, 6}, 40] (* Harvey P. Dale, Sep 23 2011 *)
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PROG
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(Magma) I:=[0, 1, 1, 6]; [n le 4 select I[n] else Self(n-1)+6*Self(n-2)+Self(n-3)-Self(n-4): n in [1..30]]; // Vincenzo Librandi, Sep 24 2011
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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Offset and name changed by T. D. Noe, Dec 22 2008
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STATUS
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approved
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