OFFSET
0,5
COMMENTS
A covering of a set is a tricovering if every element of the set is covered by exactly three blocks of the covering; A covering of a set is a T_0-covering if for every two distinct elements of the set there exists a block of the covering containing one but not the other element.
REFERENCES
I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, John Wiley and Sons, N.Y., 1983.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..15
FORMULA
E.g.f. for n-block T_0-tricoverings of a k-set is exp(-x+1/2*x^2+1/3*x^3*y)*Sum_{i=0..inf} (1+y)^binomial(i, 3)*exp(-1/2*x^2*(1+y)^i)*x^i/i!.
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladeta Jovovic, Feb 19 2001
STATUS
approved