a(n) is the 2^n-th semiprime.

Terms

a(0) =4a(1) =6a(2) =10a(3) =22a(4) =46a(5) =93a(6) =202a(7) =407a(8) =849a(9) =1774a(10) =3693a(11) =7671a(12) =15999a(13) =33146a(14) =68703a(15) =142682a(16) =295003a(17) =610757a(18) =1261573a(19) =2603453a(20) =5369633a(21) =11058907a(22) =22758881a(23) =46796443a(24) =96132103a(25) =197329777a(26) =404737537a(27) =829538129a(28) =1698995201a(29) =3477431507