Number of nonempty subsets of {1,2,...,n} in which exactly 2/3 of the elements are <= (n-4)/2.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =0a(7) =6a(8) =7a(9) =21a(10) =24a(11) =76a(12) =90a(13) =270a(14) =325a(15) =945a(16) =1155a(17) =3311a(18) =4102a(19) =11611a(20) =14547a(21) =40755a(22) =51555a(23) =143227a(24) =182703a(25) =503943a(26) =647548a(27) =1775092a(28) =2295646a(29) =6259162