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Number of n-block r-bicoverings.
12

%I #8 Jan 31 2020 14:34:25

%S 1,0,0,2,79,82117,4936900199,27555467226181396,

%T 20554872166566046969648895,2786548447182420815380482508924733911,

%U 89607283195144164483079065133414172790220498449945,864608448649084311874549352448884076627916391005243593208944730790

%N Number of n-block r-bicoverings.

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

%D I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, John Wiley and Sons, N.Y., 1983.

%H Andrew Howroyd, <a href="/A060051/b060051.txt">Table of n, a(n) for n = 0..30</a>

%F 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!.

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

%Y Column sums of A060052.

%Y Cf. A002718, A060053, A060069, A060070.

%K easy,nonn

%O 0,4

%A _Vladeta Jovovic_, Feb 15 2001

%E Terms a(11) and beyond from _Andrew Howroyd_, Jan 30 2020