Happy repdigit numbers.

Terms

a(0) =1a(1) =7a(2) =44a(3) =888a(4) =5555a(5) =88888a(6) =1111111a(7) =2222222a(8) =22222222a(9) =77777777a(10) =1111111111a(11) =7777777777a(12) =22222222222a(13) =44444444444a(14) =444444444444