Numbers k such that k concatenated with k 1's is a prime.

Terms

a(0) =1a(1) =2a(2) =5a(3) =7a(4) =10a(5) =16a(6) =20a(7) =65a(8) =91a(9) =119a(10) =169a(11) =290a(12) =428a(13) =610a(14) =905a(15) =1051a(16) =3488a(17) =4526a(18) =6445a(19) =8693a(20) =32059a(21) =111860