a(n) is the position of (prime(n+1) + prime(n+2))/2 in the ordered nonprimes.

Terms

a(0) =2a(1) =3a(2) =5a(3) =7a(4) =9a(5) =11a(6) =13a(7) =17a(8) =20a(9) =23a(10) =27a(11) =29a(12) =31a(13) =35a(14) =40a(15) =43a(16) =46a(17) =50a(18) =52a(19) =55a(20) =59a(21) =63a(22) =69a(23) =74a(24) =76a(25) =78a(26) =80a(27) =82a(28) =90a(29) =98