OFFSET
0,3
COMMENTS
For n>=5, a(n) is equal to the number of functions f:{1,2,...,n}->{1,2,3,4,5} such that for fixed, different x_1, x_2, x_3, x_4, x_5 in {1,2,...,n} and fixed y_1, y_2, y_3, y_ 4, y_5 in {1,2,3,4,5} we have f(x_i)<>y_i, (i=1,2,3,4,5). - Milan Janjic, May 13 2007
REFERENCES
C. Reutenauer, Free Lie algebras. London Mathematical Society Monographs. New Series, 7. Oxford Science Publications. The Clarendon Press, Oxford University Press, New York, 1993. xviii+269 pp.
LINKS
N. Bergeron, C. Reutenauer, M. Rosas and M. Zabrocki, Invariants and Coinvariants of the Symmetric Group in Noncommuting Variables, arXiv:math/0502082 [math.CO], 2005; Canad. J. Math. 60 (2008), no. 2, 266-296.
Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets
FORMULA
G.f.: (1-q)^5/(1-5q) sum( (-1)^k*C(5,k) 5^(n-k); k=0..min(n,5));
a(n) = 1024*5^(n-5) for n>5. - Jean-François Alcover, Dec 10 2018
MAPLE
f:=n->add((-1)^k*binomial(5, k)*5^(n-k), k=0..min(n, 4)): seq(f(i), i=0..15);
MATHEMATICA
a[n_] := If[n<6, {1, 0, 10, 40, 205, 1024}[[n+1]], 1024*5^(n-5)];
Table[a[n], {n, 0, 21}] (* Jean-François Alcover, Dec 10 2018 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Mike Zabrocki, Apr 20 2006
STATUS
approved