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A306511
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Number of permutations p of [n] having at least one index i with |p(i)-i| = 1.
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2
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0, 0, 1, 4, 19, 99, 603, 4248, 34115, 307875, 3085203, 33993870, 408482695, 5316309607, 74499953255, 1118421967520, 17907571955927, 304619809031127, 5486197279305911, 104289196264058030, 2086706157642260387, 43838287730208552691, 964790364323910060691
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OFFSET
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0,4
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LINKS
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FORMULA
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MAPLE
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a:= proc(n) option remember; `if`(n<5, [0$2, 1, 4, 19][n+1],
(2*(n^3-8*n^2+20*n-14)*a(n-1)-(n-4)*(n-1)*(n^2-5*n+7)*
a(n-2)-(n-2)*(n^2-7*n+13)*a(n-3)+(n^4-12*n^3+53*n^2
-102*n+71)*a(n-4)+(n-4)*(n^2-5*n+7)*a(n-5))/(n^2-7*n+13))
end:
seq(a(n), n=0..23);
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MATHEMATICA
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a[n_] := n! - Sum[Sum[(-1)^k (i-k)! Binomial[2i-k, k], {k, 0, i}],
{i, 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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