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A060069 Number of n-block T_0-tricoverings. 18
1, 0, 0, 0, 2, 82194, 9185157387760082, 5573096894405951375691132323893805593, 47933892393105239218152796441416602126447041437452022947424986090407628 (list; graph; refs; listen; history; text; internal format)
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

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

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

Cf. A060070, A060051-A060053, A002718, A059443, A003462, A059945-A059951.

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

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Last modified December 13 22:55 EST 2019. Contains 329974 sequences. (Running on oeis4.)