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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =2a(5) =8a(6) =20a(7) =41a(8) =63a(9) =126a(10) =252a(11) =505a(12) =1000a(13) =1936a(14) =3652a(15) =7305a(16) =14313a(17) =27573a(18) =52325a(19) =97868a(20) =180404a(21) =327692a(22) =586568a(23) =1034930a(24) =2718254a(25) =5012504a(26) =9131804a(27) =16440929a(28) =29259404a(29) =51482510