OFFSET
0,3
LINKS
FORMULA
EXAMPLE
a(2) = 5 = 6 - 1 = |{1234, 1243, 1324, 2134, 2143, 3142}| - |{1234}|.
MAPLE
a:= n-> ceil((7/9)*(2*n)!/2^n):
seq(a(n), n=0..22);
# second Maple program:
a:= proc(n) a(n):= `if`(n<4, [1$2, 5, 70][n+1], (2*n-1)*n*a(n-1)) end:
seq(a(n), n=0..22);
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Alois P. Heinz, Feb 22 2024
STATUS
approved