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A216241
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Number of n-step walks (each step +-1 starting from 0) which are never more than 5 or less than -5.
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3
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1, 2, 4, 8, 16, 32, 62, 124, 236, 472, 890, 1780, 3340, 6680, 12502, 25004, 46732, 93464, 174554, 349108, 651740, 1303480, 2432918, 4865836, 9080956, 18161912, 33892954, 67785908, 126494956, 252989912, 472095062, 944190124, 1761901676, 3523803352, 6575544410, 13151088820
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OFFSET
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0,2
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LINKS
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FORMULA
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a(n) = 6*a(n-2) - 9*a(n-4) + 2*a(n-6).
a(n) = 2^n for n < 6.
G.f.: ((1-x)^2*(1+x)^2*(1+2*x)) / ((1-2*x^2)*(1-4*x^2+x^4)).
a(2*n+1) = 2*a(2*n).
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MATHEMATICA
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nn=35; CoefficientList[Series[(1+2x)(1-x^2)^2/(1-6x^2+9x^4-2x^6), {x, 0, nn}], x] (* Geoffrey Critzer, Jan 14 2014 *)
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CROSSREFS
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KEYWORD
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nonn,walk,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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