A199495 Number of permutations of [n] starting and ending with an odd number.
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
Keywords
Links
- Alois P. Heinz, Table of n, a(n) for n = 0..450
Programs
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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
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)