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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =4a(6) =12a(7) =30a(8) =60a(9) =121a(10) =205a(11) =385a(12) =630a(13) =1260a(14) =2156a(15) =4711a(16) =8260a(17) =18340a(18) =31620a(19) =68772a(20) =115995a(21) =250185a(22) =418825a(23) =914485a(24) =1535391a(25) =3405402a(26) =5728086a(27) =12788516a(28) =21448791a(29) =47902764