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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =2a(6) =10a(7) =15a(8) =45a(9) =60a(10) =140a(11) =175a(12) =360a(13) =435a(14) =891a(15) =1071a(16) =2415a(17) =2940a(18) =7260a(19) =8910a(20) =22359a(21) =27345a(22) =66625a(23) =80795a(24) =191191a(25) =230230a(26) =541190a(27) =649805a(28) =1548755a(29) =1860131