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

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =3a(5) =9a(6) =12a(7) =15a(8) =40a(9) =51a(10) =63a(11) =175a(12) =220a(13) =270a(14) =759a(15) =945a(16) =1155a(17) =3311a(18) =4102a(19) =5005a(20) =14547a(21) =17955a(22) =21875a(23) =64275a(24) =79116a(25) =96288a(26) =285413a(27) =350568a(28) =426303a(29) =1272601