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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =2a(6) =3a(7) =15a(8) =20a(9) =60a(10) =75a(11) =175a(12) =210a(13) =435a(14) =511a(15) =1071a(16) =1260a(17) =2940a(18) =3510a(19) =8910a(20) =10725a(21) =27345a(22) =32835a(23) =80795a(24) =96382a(25) =230230a(26) =273273a(27) =649805a(28) =770315a(29) =1860131