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A176127
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The number of permutations of {1,2,...,n,1,2,...,n} with the property that there are k numbers between the two k's in the set for k=1,...,n.
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7
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0, 0, 2, 2, 0, 0, 52, 300, 0, 0, 35584, 216288, 0, 0, 79619280, 653443600, 0, 0, 513629782560, 5272675722400, 0, 0, 7598911885030976, 93690316113031872, 0, 0, 223367222197529806464, 3214766521218764786304, 0, 0
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OFFSET
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1,3
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REFERENCES
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LINKS
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FORMULA
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EXAMPLE
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a(1)=0; a(2)=0; a(3)=a(4)=2 since {{2,3,1,2,1,3},{3,1,2,1,3,2}} and {{4,1,3,1,2,4,3,2},{2,3,4,2,1,3,1,4}} are the only ways to permute {1,2,3,1,2,3} and {1,2,3,4,1,2,3,4}, respectively, such that there is one number between the 1's, two numbers between the 2's,..., n numbers between the n's.
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PROG
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(Sage) a=lambda n:sum(1 for i in DLXCPP([(i-1, j+n, i+j+n+1)for i in[1..n]for j in[0..n+n-i-2]]+[(i, )for i in[n..n+n-1]]))if n%4 in[0, 3] else 0
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CROSSREFS
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KEYWORD
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nonn,hard,more,nice
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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