Primes of the form identical digits followed by a 1.

Terms

a(0) =11a(1) =31a(2) =41a(3) =61a(4) =71a(5) =331a(6) =661a(7) =881a(8) =991a(9) =2221a(10) =3331a(11) =4441a(12) =6661a(13) =33331a(14) =99991a(15) =333331a(16) =3333331a(17) =9999991a(18) =33333331a(19) =6666666661a(20) =44444444441a(21) =555555555551