Replace each 1 in decimal expansion of n with 1 1's, each 2 with 2 2's, etc. (0 vanishes).

Terms

a(0) =1a(1) =22a(2) =333a(3) =4444a(4) =55555a(5) =666666a(6) =7777777a(7) =88888888a(8) =999999999a(9) =1a(10) =11a(11) =122a(12) =1333a(13) =14444a(14) =155555a(15) =1666666a(16) =17777777a(17) =188888888a(18) =1999999999a(19) =22a(20) =221a(21) =2222a(22) =22333a(23) =224444a(24) =2255555a(25) =22666666a(26) =227777777a(27) =2288888888a(28) =22999999999a(29) =333