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

Terms

a(0) =1a(1) =4a(2) =5a(3) =6a(4) =7a(5) =8a(6) =9a(7) =10a(8) =11a(9) =12a(10) =13a(11) =14a(12) =15a(13) =16a(14) =17a(15) =18a(16) =19a(17) =21a(18) =22a(19) =23a(20) =25a(21) =26a(22) =27a(23) =29a(24) =31a(25) =33a(26) =34a(27) =37a(28) =38a(29) =39