a(n) is the smallest composite number obtained by appending one or more 1's to n.

Terms

a(0) =111a(1) =21a(2) =3111a(3) =411a(4) =51a(5) =611a(6) =711a(7) =81a(8) =91a(9) =1011a(10) =111a(11) =121a(12) =1311a(13) =141a(14) =15111a(15) =161a(16) =171a(17) =18111a(18) =1911a(19) =201a(20) =21111a(21) =221a(22) =231a(23) =24111a(24) =2511a(25) =261a(26) =27111a(27) =2811a(28) =291a(29) =301