For n >= 6, max( prevprime(n), 2*prevprime(floor(n/2))).

Terms

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