a(n) = least positive integer > a(n-1) and not a(i)*a(j) for 1<=i<j<=n, with initial terms 2,4.

Terms

a(0) =2a(1) =4a(2) =5a(3) =6a(4) =7a(5) =9a(6) =11a(7) =13a(8) =15a(9) =16a(10) =17a(11) =19a(12) =21a(13) =23a(14) =25a(15) =27a(16) =29a(17) =31a(18) =33a(19) =37a(20) =39a(21) =40a(22) =41a(23) =43a(24) =47a(25) =48a(26) =49a(27) =51a(28) =53a(29) =56