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A126780
Number of functions f:{1,2,...,n}->{1,2,...,n} such that Im(f) contains 4 fixed elements.
2
24, 360, 5880, 109200, 2298744, 54313560, 1425878520, 41215616640, 1301388272184, 44580423767880, 1647012735721080, 65286028105225200, 2764048499876532984, 124490422185142171320, 5943580498312544073720
OFFSET
4,1
FORMULA
a(n)=n^n-4*(n-1)^n+6*(n-2)^n-4*(n-3)^n+(n-4)^n, (n=4,5,...)
EXAMPLE
a(7)=109200
MAPLE
a:=n->n^n-4*(n-1)^n+6*(n-2)^n-4*(n-3)^n+(n-4)^n;
MATHEMATICA
Drop[Table[Sum[(-1)^k Binomial[4, k] (n-k)^n, {k, 0, 4}], {n, 1, 20}], 3] (* Geoffrey Critzer, Dec 23 2012 *)
CROSSREFS
Sequence in context: A005546 A309843 A081144 * A052741 A046905 A028115
KEYWORD
nonn
AUTHOR
Aleksandar M. Janjic and Milan Janjic, Feb 18 2007
STATUS
approved