2n - {largest prime < n}.

Terms

a(0) =4a(1) =5a(2) =7a(3) =7a(4) =9a(5) =9a(6) =11a(7) =13a(8) =15a(9) =13a(10) =15a(11) =15a(12) =17a(13) =19a(14) =21a(15) =19a(16) =21a(17) =21a(18) =23a(19) =25a(20) =27a(21) =25a(22) =27a(23) =29a(24) =31a(25) =33a(26) =35a(27) =31a(28) =33a(29) =33