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A173844
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Number of permutations of 1..n with no adjacent pair summing to n+4.
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0
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1, 1, 2, 6, 24, 72, 480, 2640, 23040, 175680, 1895040, 18385920, 235791360, 2771112960, 41153495040, 567422392320, 9572600217600, 151385755852800, 2858960008396800, 50989753530777600, 1065538430749900800, 21152552961009254400, 484535856895701811200
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OFFSET
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0,3
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COMMENTS
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If a(n,k) is the number of permutations of 1..n with no adjacent pair summing to n+k, then a(n,k) = a(n,k+1) for n+k even. [proved by William Keith]
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LINKS
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FORMULA
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k = 4; a(n,k) = Sum_{j=0..m} (-2)^j*binomial(m,j)*(n-j)! where m = max(0, floor((n-k+1)/2)). [Max Alekseyev, on the Sequence Fans Mailing List]
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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