Crater numbers.

Terms

a(0) =0a(1) =1a(2) =2a(3) =3a(4) =4a(5) =5a(6) =6a(7) =7a(8) =8a(9) =9a(10) =101a(11) =212a(12) =323a(13) =434a(14) =545a(15) =656a(16) =767a(17) =878a(18) =989a(19) =21012a(20) =32123a(21) =43234a(22) =54345a(23) =65456a(24) =76567a(25) =87678a(26) =98789a(27) =3210123a(28) =4321234a(29) =5432345