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

Terms

a(0) =0a(1) =0a(2) =0a(3) =2a(4) =3a(5) =9a(6) =12a(7) =30a(8) =40a(9) =100a(10) =135a(11) =335a(12) =455a(13) =1127a(14) =1540a(15) =3822a(16) =5250a(17) =13050a(18) =18000a(19) =44802a(20) =62007a(21) =154517a(22) =214467a(23) =534963a(24) =744315a(25) =1858155a(26) =2590679a(27) =6472167a(28) =9039823a(29) =22597759