Nonnegative integers in base -4.

Terms

a(0) =0a(1) =1a(2) =2a(3) =3a(4) =130a(5) =131a(6) =132a(7) =133a(8) =120a(9) =121a(10) =122a(11) =123a(12) =110a(13) =111a(14) =112a(15) =113a(16) =100a(17) =101a(18) =102a(19) =103a(20) =230a(21) =231a(22) =232a(23) =233a(24) =220a(25) =221a(26) =222a(27) =223a(28) =210a(29) =211