a(1) = 4; a(n) is smallest semiprime > 3*a(n-1).

Terms

a(0) =4a(1) =14a(2) =46a(3) =141a(4) =427a(5) =1282a(6) =3849a(7) =11553a(8) =34663a(9) =104001a(10) =312005a(11) =936017a(12) =2808053a(13) =8424161a(14) =25272487a(15) =75817463a(16) =227452391a(17) =682357183a(18) =2047071551a(19) =6141214658a(20) =18423643982a(21) =55270931959a(22) =165812795887a(23) =497438387665