(nextprime(n)+prevprime(n))/2.

Terms

a(0) =4a(1) =5a(2) =6a(3) =8a(4) =9a(5) =9a(6) =9a(7) =10a(8) =12a(9) =14a(10) =15a(11) =15a(12) =15a(13) =16a(14) =18a(15) =20a(16) =21a(17) =21a(18) =21a(19) =24a(20) =26a(21) =26a(22) =26a(23) =26a(24) =26a(25) =27a(26) =30a(27) =33a(28) =34a(29) =34