A060051 Number of n-block r-bicoverings.

1, 0, 0, 2, 79, 82117, 4936900199, 27555467226181396, 20554872166566046969648895, 2786548447182420815380482508924733911, 89607283195144164483079065133414172790220498449945, 864608448649084311874549352448884076627916391005243593208944730790

Offset: 0

Author

Vladeta Jovovic, Feb 15 2001

Keywords

• easy
• nonn

Comments

A bicovering is an r-bicovering if the intersection of every two blocks contains at most one element.

Examples

			There are 2 3-block r-bicoverings: {{1},{2},{1,2}} and {{1,2},{1,3},{2,3}}.
		

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..30

Crossrefs

Column sums of A060052.

Cf. A002718, A060053, A060069, A060070.

Formula

E.g.f. for number of n-block r-bicoverings of a k-set is exp(-x-1/2*x^2*y)*Sum_{i=0..inf} (1+y)^binomial(i, 2)*x^i/i!.

Extensions

Terms a(11) and beyond from Andrew Howroyd, Jan 30 2020