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A304001
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Number of permutations of [n] whose up-down signature has a nonnegative total sum.
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2
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1, 1, 1, 5, 12, 93, 360, 3728, 20160, 259535, 1814400, 27820524, 239500800, 4251096402, 43589145600, 877606592736, 10461394944000, 235288904377275, 3201186852864000, 79476406782222500, 1216451004088320000, 33020655481590446318, 562000363888803840000
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OFFSET
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0,4
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COMMENTS
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The up-down signature has (+1) for each ascent and (-1) for each descent.
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LINKS
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MAPLE
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b:= proc(u, o, t) option remember; (n->
`if`(t>=n, n!, `if`(t<-n, 0,
add(b(u-j, o+j-1, t-1), j=1..u)+
add(b(u+j-1, o-j, t+1), j=1..o))))(u+o)
end:
a:= n-> `if`(n=0, 1, add(b(j-1, n-j, 0), j=1..n)):
seq(a(n), n=0..25);
# second Maple program:
a:= n-> `if`(irem(n, 2, 'r')=0, ceil(n!/2),
add(combinat[eulerian1](n, j), j=0..r)):
seq(a(n), n=0..25);
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MATHEMATICA
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Eulerian1[n_, k_] := If[k == 0, 1, If[n == 0, 0, Sum[(-1)^j (k - j + 1)^n Binomial[n + 1, j], {j, 0, k + 1}]]];
a[n_] := Module[{r, m}, {r, m} = QuotientRemainder[n, 2]; If[m == 0, Ceiling[n!/2], Sum[Eulerian1[n, j], {j, 0, r}]]];
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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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