Doublets: base-10 representation is the juxtaposition of two identical strings.

Terms

a(0) =11a(1) =22a(2) =33a(3) =44a(4) =55a(5) =66a(6) =77a(7) =88a(8) =99a(9) =1010a(10) =1111a(11) =1212a(12) =1313a(13) =1414a(14) =1515a(15) =1616a(16) =1717a(17) =1818a(18) =1919a(19) =2020a(20) =2121a(21) =2222a(22) =2323a(23) =2424a(24) =2525a(25) =2626a(26) =2727a(27) =2828a(28) =2929a(29) =3030