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A061534
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Expansion of (1-x^2)/(1-3*x-x^2+x^3).
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0
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1, 3, 9, 29, 93, 299, 961, 3089, 9929, 31915, 102585, 329741, 1059893, 3406835, 10950657, 35198913, 113140561, 363669939, 1168951465, 3757383773, 12077432845, 38820730843, 124782241601, 401090022801, 1289231579161, 4144002518683, 13320149112409
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OFFSET
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0,2
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COMMENTS
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The number of tilings of a 2*n grid using dominoes and singletons with two horizontal dominoes or one vertical domino in the two rightmost squares. - John M. Campbell, Mar 05 2011
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LINKS
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FORMULA
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a(n) = 3*a(n-1)+a(n-2)-a(n-3) for n>3. - Colin Barker, Sep 13 2014
G.f.: -(x-1)*(x+1) / (x^3-x^2-3*x+1). - Colin Barker, Sep 13 2014
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PROG
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(PARI) Vec(-(x-1)*(x+1)/(x^3-x^2-3*x+1) + O(x^100)) \\ Colin Barker, Sep 13 2014
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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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