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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =3a(6) =12a(7) =30a(8) =60a(9) =121a(10) =217a(11) =385a(12) =700a(13) =1260a(14) =2521a(15) =4960a(16) =9460a(17) =20020a(18) =38500a(19) =71071a(20) =148161a(21) =276549a(22) =503685a(23) =1071525a(24) =2006508a(25) =3681588a(26) =8013256a(27) =14981556a(28) =27412041a(29) =60042147