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1, 1, 1, 1, 4, 4, 36, 36, 576, 576, 14400, 14400, 518400, 518400, 25401600, 25401600, 1625702400, 1625702400, 131681894400, 131681894400, 13168189440000, 13168189440000, 1593350922240000, 1593350922240000, 229442532802560000, 229442532802560000
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OFFSET
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0,5
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COMMENTS
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a(n+1) is the number of permutations of {1,2,...,n} with no even entry followed by a smaller entry. Example: a(5)=4 because we have 1234, 1324, 3124 and 2314.
a(n+1) is the number of permutations p of {1,2,...,n} such that p(j) is odd whenever j is even. Example: a(5)=4 because we have 4123, 2143, 2341 and 4321.
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LINKS
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FORMULA
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MAPLE
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MATHEMATICA
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f[n_] := 2^(n - Mod[n, 2])*Product[k^((-1)^(k+1)), {k, n}]; Array[ #!/f@# &, 25, 0]
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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