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A306675
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Number of permutations p of [2n] having at least one index i with |p(i)-i| = n.
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3
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0, 1, 15, 455, 25487, 2293839, 302786759, 55107190151, 13225725636255, 4047072044694047, 1537887376983737879, 710503968166486900119, 392198190427900768865711, 254928823778135499762712175, 192726190776270437820610404327, 167671785975355280903931051764519
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OFFSET
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0,3
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LINKS
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FORMULA
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MAPLE
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b:= proc(n, k) b(n, k):= `if`(k=0, n!, b(n+1, k-1) -b(n, k-1)) end:
a:= n-> (2*n)! -b(0, 2*n):
seq(a(n), n=0..16);
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MATHEMATICA
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b[n_, k_] := b[n, k] = If[k == 0, n!, b[n + 1, k - 1] - b[n, k - 1]];
a[n_] := (2n)! - b[0, 2n];
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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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