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A199495
Number of permutations of [n] starting and ending with an odd number.
2
0, 1, 0, 2, 4, 36, 144, 1440, 8640, 100800, 806400, 10886400, 108864000, 1676505600, 20118067200, 348713164800, 4881984307200, 94152554496000, 1506440871936000, 32011868528640000, 576213633515520000, 13380961044971520000, 267619220899430400000
OFFSET
0,4
LINKS
FORMULA
From Alois P. Heinz, Nov 07 2011: (Start)
a(n) = 2*(n-2)! * [x^(n-2)] x/((1+x)^2*(1-x)^3) for n>=2, else a(n) = n.
a(n) = 2*A152887(n-2) for n>=2, else a(n) = n. (End)
MAPLE
a:= n-> `if`(n<2, n, ceil(n/2)*(ceil(n/2)-1)*(n-2)!):
seq(a(n), n=0..30); # Alois P. Heinz, Nov 07 2011
CROSSREFS
KEYWORD
nonn
AUTHOR
Olivier Gérard, Nov 07 2011
STATUS
approved