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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =3a(5) =4a(6) =12a(7) =15a(8) =40a(9) =51a(10) =135a(11) =175a(12) =455a(13) =596a(14) =1540a(15) =2037a(16) =5250a(17) =7000a(18) =18000a(19) =24156a(20) =62007a(21) =83667a(22) =214467a(23) =290719a(24) =744315a(25) =1012935a(26) =2590679a(27) =3537703a(28) =9039823a(29) =12381252