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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =2a(6) =10a(7) =15a(8) =45a(9) =105a(10) =140a(11) =286a(12) =558a(13) =720a(14) =1500a(15) =3300a(16) =4455a(17) =10415a(18) =23855a(19) =32487a(20) =73255a(21) =160615a(22) =216580a(23) =476910a(24) =1047438a(25) =1418514a(26) =3181482a(27) =7110522a(28) =9690000a(29) =21851202