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A084481
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Number of fault-free tilings of a 4 X 2n rectangle with L tetrominoes.
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6
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2, 6, 10, 18, 38, 84, 186, 410, 904, 1994, 4398, 9700, 21394, 47186, 104072, 229538, 506262, 1116596, 2462730, 5431722, 11980040, 26422810, 58277342, 128534724, 283492258, 625261858, 1379058440, 3041609138, 6708480134, 14796018708, 32633646554, 71975773242
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OFFSET
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1,1
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COMMENTS
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Fault-free tilings are those where the only straight interface is at the left and right end. Thus a(n) <= A084480(n).
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LINKS
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FORMULA
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G.f.: 2*z*(1+z)^2*(1-z-z^3) / (1-2*z-z^3).
a(n) = 2*a(n-1) + a(n-3) for n>6. - Colin Barker, Mar 28 2017
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PROG
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(PARI) Vec(2*x*(1 + x)^2*(1 - x - x^3) / (1 - 2*x - x^3) + O(x^30)) \\ Colin Barker, Mar 28 2017
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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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