Number of nonempty subsets of {1,2,...,n} in which exactly 1/5 of the elements are <= (n-3)/3.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =5a(6) =15a(7) =35a(8) =70a(9) =141a(10) =261a(11) =405a(12) =765a(13) =1485a(14) =2310a(15) =4954a(16) =10634a(17) =16575a(18) =35945a(19) =75725a(20) =113815a(21) =240385a(22) =502945a(23) =744345a(24) =1595433a(25) =3411849a(26) =5036862a(27) =10959446a(28) =23631286a(29) =34636245