Numbers in base 4.

Terms

a(0) =0a(1) =1a(2) =2a(3) =3a(4) =10a(5) =11a(6) =12a(7) =13a(8) =20a(9) =21a(10) =22a(11) =23a(12) =30a(13) =31a(14) =32a(15) =33a(16) =100a(17) =101a(18) =102a(19) =103a(20) =110a(21) =111a(22) =112a(23) =113a(24) =120a(25) =121a(26) =122a(27) =123a(28) =130a(29) =131