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A135812
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Number of coincidence-free length n lists of 6-tuples with all numbers 1,...,n in tuple position k, for k=1..6.
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3
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OFFSET
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0,3
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COMMENTS
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a(n) enumerates (ordered) lists of n 6-tuples such that every number from 1 to n appears once at each of the six tuple positions and the j-th list member is not the tuple (j,j,j,j,j,j), for every j=1,..,n. Called coincidence-free 6-tuple lists of length n. See the Charalambides reference for this combinatorial interpretation.
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REFERENCES
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Ch. A. Charalambides, Enumerative Combinatorics, Chapman & Hall/CRC, Boca Raton, Florida, 2002, p. 187, Exercise 13.(a), for r=6.
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LINKS
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FORMULA
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a(n) = Sum_{j=0..n} ((-1)^(n-j))*binomial(n,j)*(j!)^6. See the Charalambides reference a(n)=B_{n,6}.
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EXAMPLE
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6-tuple combinatorics: a(1)=0 because the only list of 6-tuples composed of 1 is [(1,1,1,1,1,1)] and this is a coincidence for j=1.
6-tuple combinatorics: from the 2^6=64 possible 6-tuples of numbers 1 and 2 all except (1,1,1,1,1,1) appear as first members of the length 2 lists. The second members are the 6-tuples obtained by interchanging 1 and 2 in the first member. E.g. one of the a(2)=2^6-1 =63 lists is [(1,1,1,1,1,2),(2,2,2,2,2,1)]. The list [(1,1,1,1,1,1),(2,2,2,2,2,2) does not qualify because it has in fact two coincidences, those for j=1 and j=2.
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MATHEMATICA
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Table[Sum[(-1)^(n - k)*Binomial[n, k]*(k!)^6, {k, 0, n}], {n, 0, 25}] (* G. C. Greubel, Nov 23 2016 *)
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CROSSREFS
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Cf. A135811 (coincidence-free 5-tuples). A135813 (coincidence-free 7-tuples).
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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