a(n) is largest semiprime < 2*a(n-1), with a(1) = 4.

Terms

a(0) =4a(1) =6a(2) =10a(3) =15a(4) =26a(5) =51a(6) =95a(7) =187a(8) =371a(9) =737a(10) =1473a(11) =2942a(12) =5878a(13) =11755a(14) =23507a(15) =47013a(16) =94021a(17) =188041a(18) =376069a(19) =752135a(20) =1504261a(21) =3008503a(22) =6017001a(23) =12034001a(24) =24068001a(25) =48135995a(26) =96271987a(27) =192543973a(28) =385087943a(29) =770175883