a(1) = 4, a(n+1) is the largest composite number < 2a(n).

Terms

a(0) =4a(1) =6a(2) =10a(3) =18a(4) =35a(5) =69a(6) =136a(7) =270a(8) =539a(9) =1077a(10) =2152a(11) =4303a(12) =8605a(13) =17208a(14) =34415a(15) =68829a(16) =137657a(17) =275313a(18) =550625a(19) =1101249a(20) =2202497a(21) =4404993a(22) =8809985a(23) =17619969a(24) =35239936a(25) =70479871a(26) =140959741a(27) =281919481a(28) =563838961a(29) =1127677921