A126778 Number of functions f:{1,2,...,n}->{1,2,...,n} such that Im(f) contain two fixed elements.

2, 12, 110, 1320, 19502, 341796, 6927230, 159338640, 4100173022, 116692730220, 3639243694814, 123411407978040, 4521172518708302, 177943592279325684, 7487880700093141502, 335470576863324471840, 15942605654030732807102, 801021400634789561255868

Offset

2,1

Links

Table of n, a(n) for n=2..19.

Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets

Formula

a(n)=n^n-2*(n-1)^n+(n-2)^n (n=2,3,...)

Maple

a:=n->n^n-2*(n-1)^n+(n-2)^n;

Mathematica

Drop[Table[Sum[(-1)^k Binomial[2, k] (n-k)^n, {k, 0, 2}], {n, 1, 20}], 1]  (* Geoffrey Critzer, Dec 23 2012 *)

Crossrefs

Sequence in context: A217800 A241958 A217802 * A158832 A372158 A264916

Adjacent sequences: A126775 A126776 A126777 * A126779 A126780 A126781

Keyword

nonn

Author

Aleksandar M. Janjic and Milan Janjic, Feb 18 2007

Extensions

Entries checked by N. J. A. Sloane, Mar 08 2007

Status

approved