%I A059950
%S A059950 0,0,0,0,0,15,8456,954213,66253552,3622342095,172672602432,
%T A059950 7557346901841,312733696544984,12456923582109435,483124650731622328,
%U A059950 18383758048494864909,689931203330381971296,25630900118611348761735
%N A059950 Number of 9-block bicoverings of an n-set.
%D A059950 I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, John Wiley 
               and Sons, N.Y., 1983.
%F A059950 a(n)=(1/9!)*(36^n-9*28^n-36*22^n+72*21^n+252*16^n-336*15^n+378*12^n-1512*11^n+1260*10^n-1890*8^n+5040*7^n-453\
               6*6^n+7560*5^n-8820*4^n-11256*3^n+28728*2^n-19152). E.g.f. for m-block 
               bicoverings of an n-set is exp(-x-1/2*x^2*(exp(y)-1))*Sum_{i=0..inf} 
               x^i/i!*exp(binomial(i, 2)*y).
%Y A059950 Cf. A002718, A059443, A003462, A059945-A059949, A059951.
%Y A059950 Sequence in context: A066968 A113795 A074488 this_sequence A140285 A112614 
               A068732
%Y A059950 Adjacent sequences: A059947 A059948 A059949 this_sequence A059951 A059952 
               A059953
%K A059950 easy,nonn
%O A059950 1,6
%A A059950 Vladeta Jovovic (vladeta(AT)eunet.rs), Feb 14 2001