

A060069


Number of nblock T_0tricoverings.


18




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_0covering 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

Table of n, a(n) for n=0..8.


FORMULA

E.g.f. for nblock T_0tricoverings of a kset 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

Cf. A060070, A060051A060053, A002718, A059443, A003462, A059945A059951.
Sequence in context: A195003 A071067 A321246 * A231612 A296104 A170995
Adjacent sequences: A060066 A060067 A060068 * A060070 A060071 A060072


KEYWORD

nonn


AUTHOR

Vladeta Jovovic, Feb 19 2001


STATUS

approved



