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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =0a(7) =20a(8) =35a(9) =105a(10) =168a(11) =364a(12) =588a(13) =1260a(14) =2250a(15) =4960a(16) =9460a(17) =20020a(18) =38500a(19) =77001a(20) =148161a(21) =286689a(22) =560469a(23) =1071525a(24) =2143051a(25) =4064071a(26) =8259196a(27) =15473436a(28) =31705476a(29) =58620692