OFFSET
1,6
REFERENCES
I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, John Wiley and Sons, N.Y., 1983.
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..100
FORMULA
a(n) = (1/8!)*(28^n - 8*21^n - 28*16^n + 56*15^n + 168*11^n - 224*10^n + 210*8^n - 840*7^n + 700*6^n - 840*5^n + 1925*4^n + 1064*3^n - 5460*2^n + 4368).
E.g.f. for m-block bicoverings of an n-set is exp(-x-1/2*x^2*(exp(y)-1))*Sum_{i=0..inf} x^i/i!*exp(binomial(i, 2)*y).
G.f.: -5*x^6*(3390266880*x^8 -3368778336*x^7 +1334596314*x^6 -268312855*x^5 +27919999*x^4 -1171492*x^3 -29534*x^2 +4331*x -107) / ((x -1)*(2*x -1)*(3*x- 1)*(4*x -1)*(5*x -1)*(6*x -1)*(7*x -1)*(8*x -1)*(10*x -1)*(11*x -1)*(15*x -1)*(16*x -1)*(21*x -1)*(28*x -1)). - Colin Barker, Jul 08 2013
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Vladeta Jovovic, Feb 14 2001
STATUS
approved