OFFSET
0,4
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..450
Wikipedia, Derangement.
FORMULA
EXAMPLE
a(3) = 2: (123), (132).
a(4) = 9: (12)(34), (13)(24), (14)(23), (1234), (1243), (1324), (1342), (1423), (1432).
MAPLE
b:= proc(n, t) option remember; `if`(n=0, t, add(b(n-j,
`if`(j=2, 1-t, t))*binomial(n-1, j-1)*(j-1)!, j=2..n))
end:
a:= n-> b(n, 1):
seq(a(n), n=0..27);
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Jan 27 2022
STATUS
approved