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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =2a(5) =3a(6) =12a(7) =16a(8) =40a(9) =50a(10) =110a(11) =135a(12) =315a(13) =392a(14) =980a(15) =1232a(16) =3080a(17) =3864a(18) =9480a(19) =11850a(20) =29040a(21) =36300a(22) =89870a(23) =112519a(24) =280423a(25) =351351a(26) =876603a(27) =1098188a(28) =2741102a(29) =3433703