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A126781
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Number of functions f:{1,2,...,n}->{1,2,...,n} such that Im(f) contains 6 fixed elements.
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2
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720, 20160, 514080, 13608000, 385363440, 11760819840, 386860668480, 13682898028800, 518666099711760, 20997894426949440, 904827327153291360, 41367795437773022400, 2000634709955550047280, 102066613831917982920960
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OFFSET
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6,1
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LINKS
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FORMULA
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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,...)
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EXAMPLE
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a(7)=20160
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MAPLE
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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;
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MATHEMATICA
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Drop[Table[Sum[(-1)^k Binomial[6, k] (n-k)^n, {k, 0, 6}], {n, 1, 20}], 5] (* Geoffrey Critzer, Dec 23 2012 *)
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CROSSREFS
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Cf. A039621 (Lehmer-Comtet numbers of 2nd kind).
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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