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

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =2a(5) =8a(6) =12a(7) =30a(8) =40a(9) =86a(10) =110a(11) =245a(12) =315a(13) =756a(14) =980a(15) =2387a(16) =3080a(17) =7400a(18) =9480a(19) =22737a(20) =29040a(21) =70390a(22) =89870a(23) =219791a(24) =280423a(25) =688051a(26) =876603a(27) =2154347a(28) =2741102a(29) =6754580