Integers written in factorial base.

Terms

a(0) =0a(1) =1a(2) =10a(3) =11a(4) =20a(5) =21a(6) =100a(7) =101a(8) =110a(9) =111a(10) =120a(11) =121a(12) =200a(13) =201a(14) =210a(15) =211a(16) =220a(17) =221a(18) =300a(19) =301a(20) =310a(21) =311a(22) =320a(23) =321a(24) =1000a(25) =1001a(26) =1010a(27) =1011a(28) =1020a(29) =1021