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) =0a(6) =3a(7) =15a(8) =20a(9) =60a(10) =140a(11) =175a(12) =360a(13) =720a(14) =891a(15) =1935a(16) =4455a(17) =5775a(18) =13895a(19) =32487a(20) =42484a(21) =97188a(22) =216580a(23) =282165a(24) =632706a(25) =1418514a(26) =1863880a(27) =4259520a(28) =9690000a(29) =12833436