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

Terms

a(0) =0a(1) =0a(2) =0a(3) =2a(4) =3a(5) =4a(6) =12a(7) =15a(8) =18a(9) =51a(10) =63a(11) =76a(12) =220a(13) =270a(14) =325a(15) =945a(16) =1155a(17) =1390a(18) =4102a(19) =5005a(20) =6027a(21) =17955a(22) =21875a(23) =26348a(24) =79116a(25) =96288a(26) =116010a(27) =350568a(28) =426303a(29) =513729