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A275521 Number of (n+floor(n/2))-block bicoverings of an n-set.

%I A275521 #7 Jul 31 2016 17:31:25
%S A275521 1,0,1,4,3,40,15,420,105,5040,945,69300,10395,1081080,135135,18918900,
%T A275521 2027025,367567200,34459425,7856748900,654729075,183324141000,
%U A275521 13749310575,4638100767300,316234143225,126493657290000,7905853580625,3699939475732500,213458046676875
%N A275521 Number of (n+floor(n/2))-block bicoverings of an n-set.
%C A275521 There are no bicoverings of an n-set with more than n+floor(n/2) blocks.
%H A275521 Alois P. Heinz, <a href="/A275521/b275521.txt">Table of n, a(n) for n = 0..808</a>
%F A275521 a(n) = A059443(n,n+floor(n/2)).
%e A275521 a(2) = 1: 1|12|2.
%e A275521 a(3) = 4: 1|12|23|3, 1|13|2|23, 1|123|2|3, 12|13|2|3.
%e A275521 a(4) = 3: 1|12|2|3|34|4, 1|13|2|24|3|4, 1|14|2|23|3|4.
%p A275521 a:= proc(n) option remember; `if`(n<5, [1, 0, 1, 4, 3]
%p A275521        [n+1], ((8*n-41)*a(n-1) +(6*n^2-12*n-12)*a(n-2)
%p A275521        -(n-2)*(8*n-17)*a(n-3)) / (6*n-24))
%p A275521     end:
%p A275521 seq(a(n), n=0..30);
%Y A275521 Right border of triangle A059443.
%Y A275521 Bisections give: A001147, 4*A000457(n-1) (for n>0).
%K A275521 nonn
%O A275521 0,4
%A A275521 _Alois P. Heinz_, Jul 31 2016