a(1) = 4; a(n) is smallest semiprime > 2*a(n-1).

Terms

a(0) =4a(1) =9a(2) =21a(3) =46a(4) =93a(5) =187a(6) =377a(7) =755a(8) =1513a(9) =3027a(10) =6059a(11) =12127a(12) =24257a(13) =48529a(14) =97059a(15) =194127a(16) =388257a(17) =776515a(18) =1553033a(19) =3106083a(20) =6212177a(21) =12424355a(22) =24848723a(23) =49697447a(24) =99394909a(25) =198789819a(26) =397579639a(27) =795159283a(28) =1590318573a(29) =3180637153