Integers k such that 10^k+21 is prime.

Terms

a(0) =1a(1) =3a(2) =9a(3) =17a(4) =55a(5) =77a(6) =133a(7) =195a(8) =357a(9) =1537a(10) =2629a(11) =3409a(12) =8007a(13) =25671a(14) =48003a(15) =55811a(16) =94983a(17) =109615a(18) =135669