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A061703
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G.f.: 2*x*(2-2*x-3*x^2+2*x^3)/((1-3*x-x^2+x^3)*(1-x)).
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2
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0, 4, 12, 34, 108, 344, 1104, 3546, 11396, 36628, 117732, 378426, 1216380, 3909832, 12567448, 40395794, 129844996, 417363332, 1341539196, 4312135922, 13860583628, 44552347608, 143205490528, 460308235562, 1479577849604
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OFFSET
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0,2
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REFERENCES
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R. P. Stanley, Enumerative Combinatorics I, Example 4.7.17.
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LINKS
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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