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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =2a(5) =8a(6) =12a(7) =30a(8) =63a(9) =86a(10) =182a(11) =392a(12) =560a(13) =1270a(14) =2800a(15) =4070a(16) =9020a(17) =19595a(18) =28679a(19) =63518a(20) =138866a(21) =205478a(22) =455966a(23) =997640a(24) =1485624a(25) =3290520a(26) =7201692a(27) =10782222a(28) =23878212a(29) =52311408