Numbers that contain only 1's, 2's and 3's.

Terms

a(0) =1a(1) =2a(2) =3a(3) =11a(4) =12a(5) =13a(6) =21a(7) =22a(8) =23a(9) =31a(10) =32a(11) =33a(12) =111a(13) =112a(14) =113a(15) =121a(16) =122a(17) =123a(18) =131a(19) =132a(20) =133a(21) =211a(22) =212a(23) =213a(24) =221a(25) =222a(26) =223a(27) =231a(28) =232a(29) =233