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

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =6a(5) =13a(6) =25a(7) =45a(8) =91a(9) =175a(10) =322a(11) =645a(12) =1245a(13) =2325a(14) =4651a(15) =9031a(16) =17061a(17) =34123a(18) =66547a(19) =126763a(20) =253527a(21) =496063a(22) =950818a(23) =1901637a(24) =3730293a(25) =7184421a(26) =14368843a(27) =28243063a(28) =54604081a(29) =109208163