Number of nonempty subsets of {1,2,...,n} in which exactly 1/5 of the elements are <= (n-4)/2.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =5a(6) =15a(7) =30a(8) =70a(9) =105a(10) =213a(11) =286a(12) =558a(13) =720a(14) =1500a(15) =1935a(16) =4455a(17) =5775a(18) =13895a(19) =17876a(20) =42484a(21) =53859a(22) =124761a(23) =156065a(24) =357825a(25) =444015a(26) =1028500a(27) =1272155a(28) =3002795a(29) =3709043