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A286911
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Number of edge covers in the ladder graph P_2 x P_n.
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5
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1, 7, 43, 277, 1777, 11407, 73219, 469981, 3016729, 19363879, 124293499, 797819173, 5121067777, 32871277183, 210995228083, 1354343064493, 8693301516841, 55800847838359, 358176305451691, 2299073773191541, 14757369859827601, 94725087867636847
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OFFSET
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1,2
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LINKS
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FORMULA
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a(n) = 6*a(n-1) + 3*a(n-2) - 2*a(n-3) for n > 3.
G.f.: x*(1-x)*(1+2*x)/(1-6*x-3*x^2+2*x^3).
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MATHEMATICA
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Table[-RootSum[2 - 3 # - 6 #^2 + #^3 &, -14 #^n - 5 #^(n + 1) + #^(n + 2) &]/30, {n, 20}] (* Eric W. Weisstein, Aug 09 2017 *)
CoefficientList[Series[(1 + x - 2 x^2)/(1 - 6 x - 3 x^2 + 2 x^3), {x, 0, 20}], x] (* Eric W. Weisstein, Aug 09 2017 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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