Number of 4-block bicoverings of an n-set.

Terms

a(0) =0a(1) =0a(2) =4a(3) =39a(4) =280a(5) =1815a(6) =11284a(7) =68859a(8) =416560a(9) =2509455a(10) =15086764a(11) =90610179a(12) =543928840a(13) =3264374295a(14) =19588645444a(15) =117539063499a(16) =705255937120