a(2n) = a(n), a(2n+1) = 10*a(n) + a(n+1).

Terms

a(0) =1a(1) =1a(2) =11a(3) =1a(4) =21a(5) =11a(6) =111a(7) =1a(8) =31a(9) =21a(10) =221a(11) =11a(12) =221a(13) =111a(14) =1111a(15) =1a(16) =41a(17) =31a(18) =331a(19) =21a(20) =431a(21) =221a(22) =2221a(23) =11a(24) =331a(25) =221a(26) =2321a(27) =111a(28) =2221a(29) =1111