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A126781
Number of functions f:{1,2,...,n}->{1,2,...,n} such that Im(f) contains 6 fixed elements.
2
720, 20160, 514080, 13608000, 385363440, 11760819840, 386860668480, 13682898028800, 518666099711760, 20997894426949440, 904827327153291360, 41367795437773022400, 2000634709955550047280, 102066613831917982920960
OFFSET
6,1
FORMULA
a(n)=n^n-6*(n-1)^n+15*(n-2)^n-20*(n-3)^n+15*(n-4)^n-6*(n-5)^n+(n-6)^n, (n=6,7,...)
EXAMPLE
a(7)=20160
MAPLE
a:=n->n^n-6*(n-1)^n+15*(n-2)^n-20*(n-3)^n+15*(n-4)^n-6*(n-5)^n+(n-6)^n;
MATHEMATICA
Drop[Table[Sum[(-1)^k Binomial[6, k] (n-k)^n, {k, 0, 6}], {n, 1, 20}], 5] (* Geoffrey Critzer, Dec 23 2012 *)
CROSSREFS
Cf. A039621 (Lehmer-Comtet numbers of 2nd kind).
Sequence in context: A254079 A037212 A228909 * A090008 A223857 A003938
KEYWORD
nonn
AUTHOR
Aleksandar M. Janjic and Milan Janjic, Feb 18 2007
STATUS
approved