Number of nonempty subsets of {1,2,...,n} in which exactly 1/4 of the elements are <= n/3.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =4a(5) =8a(6) =20a(7) =41a(8) =63a(9) =126a(10) =252a(11) =392a(12) =844a(13) =1780a(14) =2800a(15) =5995a(16) =12545a(17) =19595a(18) =41951a(19) =88634a(20) =138866a(21) =299411a(22) =636524a(23) =997640a(24) =2154104a(25) =4594244a(26) =7201692a(27) =15584085a(28) =33355944a(29) =52311408