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A229885
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Number of 4 up, 4 down permutations of [n].
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3
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1, 1, 1, 1, 1, 1, 5, 15, 35, 70, 574, 2674, 9274, 26599, 305747, 1944033, 8995805, 33757360, 498851248, 4017418768, 23236611280, 107709888805, 1945409895065, 18965460022971, 131635127294783, 726401013530416, 15505381392117616, 177447751441161616
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OFFSET
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0,7
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COMMENTS
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Limit n->infinity (a(n)/n!)^(1/n) = 0.38605986196... . - Vaclav Kotesovec, Sep 06 2014
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LINKS
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EXAMPLE
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a(5) = 1: 12345.
a(6) = 5: 123465, 123564, 124563, 134562, 234561.
a(7) = 15: 1234765, 1235764, 1236754, 1245763, 1246753, 1256743, 1345762, 1346752, 1356742, 1456732, 2345761, 2346751, 2356741, 2456731, 3456721.
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MAPLE
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b:= proc(u, o, t) option remember; `if`(u+o=0, 1, add(`if`(t=4,
b(o-j, u+j-1, 1), b(u+j-1, o-j, t+1)), j=1..o))
end:
a:= n-> b(0, n, 0):
seq(a(n), n=0..30);
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MATHEMATICA
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b[u_, o_, t_] := b[u, o, t] = If[u + o == 0, 1, Sum[If[t == 4, b[o - j, u + j - 1, 1], b[u + j - 1, o - j, t + 1]], {j, 1, o}]];
a[n_] := b[0, n, 0];
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CROSSREFS
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KEYWORD
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nonn,eigen
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AUTHOR
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STATUS
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approved
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