a(n) = j if n is the j-th prime, else a(n) = k if n is the k-th nonprime.

Terms

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