a(n) is the least composite number obtained by inserting a nonempty string of 0's inside n.

Terms

a(0) =100a(1) =1001a(2) =102a(3) =1003a(4) =104a(5) =105a(6) =106a(7) =1007a(8) =108a(9) =100009a(10) =200a(11) =201a(12) =202a(13) =203a(14) =204a(15) =205a(16) =206a(17) =207a(18) =208a(19) =209a(20) =300a(21) =301a(22) =302a(23) =303a(24) =304a(25) =305a(26) =306a(27) =3007a(28) =308a(29) =309