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A173849
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Number of permutations of 1..n with no adjacent pair summing to n + 9.
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0
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1, 1, 2, 6, 24, 120, 720, 5040, 40320, 362880, 2903040, 32659200, 333849600, 4470681600, 55244851200, 855496857600, 12362073292800, 216999220838400, 3584572069478400, 70276861528473600, 1305169212624076800, 28251875803447296000, 582386191297118208000
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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.
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LINKS
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FORMULA
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k = 9; m = max(0, floor((n-k+1)/2)); a(n,k) = Sum_{j=0..m} (-2)^j * binomial(m,j)*(n-j)!.
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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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Comment proved by William Keith, formula from Max Alekseyev, on the Sequence Fans Mailing List
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STATUS
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approved
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