

A060070


Number of T_0tricoverings of an nset.


21



1, 0, 0, 5, 175, 9426, 751365, 84012191, 12644839585, 2479642897109, 617049443550205, 190678639438170502, 71860665148118443795, 32527628234581386962713, 17454341903042193018433239
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OFFSET

0,4


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..14.
T_0tricoverings of a 4set


FORMULA

E.g.f. for kblock T_0tricoverings of an nset 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. A060069, A060051A060053, A002718, A059443, A003462, A059945A059951.
Sequence in context: A139986 A123111 A303154 * A300590 A027873 A203529
Adjacent sequences: A060067 A060068 A060069 * A060071 A060072 A060073


KEYWORD

nonn


AUTHOR

Vladeta Jovovic, Feb 21 2001


STATUS

approved



