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A060070 Number of T_0-tricoverings of an n-set. 21
1, 0, 0, 5, 175, 9426, 751365, 84012191, 12644839585, 2479642897109, 617049443550205, 190678639438170502, 71860665148118443795, 32527628234581386962713, 17454341903042193018433239 (list; graph; refs; listen; history; text; internal format)
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_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

Table of n, a(n) for n=0..14.

T_0-tricoverings of a 4-set

FORMULA

E.g.f. for k-block T_0-tricoverings of an n-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

Cf. A060069, A060051-A060053, A002718, A059443, A003462, A059945-A059951.

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

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Last modified March 23 14:17 EDT 2019. Contains 321431 sequences. (Running on oeis4.)