3*n analog to Keith numbers.

Terms

a(0) =7a(1) =9a(2) =14a(3) =19a(4) =21a(5) =28a(6) =38a(7) =53a(8) =54a(9) =76a(10) =92a(11) =124a(12) =1299a(13) =18185a(14) =20468a(15) =31871a(16) =32054a(17) =37903a(18) =128200a(19) =152057a(20) =175539a(21) =193399a(22) =214631a(23) =303677a(24) =1806425a(25) =3250457a(26) =3616693a(27) =7870170a(28) =10793441a(29) =12047403