OFFSET
2,2
FORMULA
a(n) = (n-1)! * (n^2 - 3*n + 3) (n>=2).
a(n) = A001564(n-2)*(n-1) for n>=2. - Anton Zakharov, Sep 14 2016
D-finite with recurrence a(n) +(-n-5)*a(n-1) +(4*n-1)*a(n-2) +3*(-n+3)*a(n-3)=0. - R. J. Mathar, Jul 22 2022
EXAMPLE
a(3)=6 because in 123, 132, 213, 231, 312, 321 we have 0+1+1+2+2+0 such pairs.
MAPLE
seq(factorial(n-1)*(n^2-3*n+3), n = 2 .. 20);
MATHEMATICA
Table[(n-1)!(n^2-3n+3), {n, 2, 30}] (* Harvey P. Dale, Mar 28 2012 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Emeric Deutsch, Jul 19 2009
STATUS
approved