Number of nonempty subsets of {1,2,...,n} in which exactly 2/5 of the elements are <= (n+1)/3.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =4a(6) =10a(7) =30a(8) =60a(9) =105a(10) =217a(11) =364a(12) =588a(13) =1260a(14) =2250a(15) =3960a(16) =9460a(17) =17380a(18) =30745a(19) =71071a(20) =126763a(21) =219765a(22) =503685a(23) =898380a(24) =1572228a(25) =3681588a(26) =6619596a(27) =11666760a(28) =27412041a(29) =49146654