Number of nonempty subsets of {1,2,...,n} in which exactly 2/5 of the elements are <= n/2.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =3a(6) =12a(7) =24a(8) =60a(9) =100a(10) =205a(11) =315a(12) =630a(13) =980a(14) =2156a(15) =3528a(16) =8260a(17) =13692a(18) =31620a(19) =51600a(20) =115995a(21) =186945a(22) =418825a(23) =675675a(24) =1535391a(25) =2492919a(26) =5728086a(27) =9324406a(28) =21448791a(29) =34860553