Twice repdigit numbers.

Terms

a(0) =0a(1) =2a(2) =4a(3) =6a(4) =8a(5) =10a(6) =12a(7) =14a(8) =16a(9) =18a(10) =22a(11) =44a(12) =66a(13) =88a(14) =110a(15) =132a(16) =154a(17) =176a(18) =198a(19) =222a(20) =444a(21) =666a(22) =888a(23) =1110a(24) =1332a(25) =1554a(26) =1776a(27) =1998a(28) =2222a(29) =4444