a(n) = 2n - {smallest prime > n}.

Terms

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