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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =4a(6) =5a(7) =15a(8) =18a(9) =51a(10) =63a(11) =175a(12) =220a(13) =596a(14) =759a(15) =2037a(16) =2625a(17) =7000a(18) =9108a(19) =24156a(20) =31683a(21) =83667a(22) =110487a(23) =290719a(24) =386155a(25) =1012935a(26) =1352263a(27) =3537703a(28) =4743700a(29) =12381252