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A217525
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From the enumeration of involutions avoiding the pattern 4321.
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1
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0, 0, 0, 0, 0, 2, 4, 6, 15, 31, 67, 155, 343, 787, 1829, 4233, 9940, 23454, 55540, 132390, 316704, 760462, 1833050, 4431552, 10745282, 26125316, 63672044, 155536070, 380739734, 933832952, 2294581836, 5647750518, 13923153431, 34375447863, 84989987987, 210407135915, 521547749731, 1294310608775, 3215632551785, 7997467696213, 19910095233857
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OFFSET
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0,6
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LINKS
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FORMULA
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Manara and Cippo give a g.f.
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MAPLE
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local x ;
-4+4/(1+x^2)+1/(1+x)^2 ;
1/(1+x)-2*x^2*(1+x)-sqrt(%) ;
taylor(%/2, x=0, nmax+1) ;
seq(coeftayl(%, x=0, n), n=0..nmax) ;
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MATHEMATICA
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a[n_] := SeriesCoefficient[(1/2)(-2(x+1)x^2 - Sqrt[4/(x^2+1) + 1/(x+1)^2 - 4] + 1/(x+1)), {x, 0, n}];
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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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