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

Terms

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