a(n+1) = 2a(n) - {largest prime < a(n)}.

Terms

a(0) =3a(1) =4a(2) =5a(3) =7a(4) =9a(5) =11a(6) =15a(7) =17a(8) =21a(9) =23a(10) =27a(11) =31a(12) =33a(13) =35a(14) =39a(15) =41a(16) =45a(17) =47a(18) =51a(19) =55a(20) =57a(21) =61a(22) =63a(23) =65a(24) =69a(25) =71a(26) =75a(27) =77a(28) =81a(29) =83