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

Terms

a(0) =0a(1) =0a(2) =0a(3) =2a(4) =6a(5) =9a(6) =21a(7) =45a(8) =70a(9) =154a(10) =322a(11) =525a(12) =1125a(13) =2325a(14) =3921a(15) =8301a(16) =17061a(17) =29491a(18) =61915a(19) =126763a(20) =223210a(21) =465746a(22) =950818a(23) =1698453a(24) =3527109a(25) =7184421a(26) =12981421a(27) =26855641a(28) =54604081a(29) =99589963