A060069 Number of n-block T_0-tricoverings.

1, 0, 0, 0, 2, 82194, 9185157387760082, 5573096894405951375691132323893805593, 47933892393105239218152796441416602126447041437452022947424986090407628

Offset: 0

Author

Vladeta Jovovic, Feb 19 2001

Keywords

• nonn

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

Crossrefs

Column sums of A059530.

Cf. A060051, A060053, A060070, A060486.

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!.