Numbers k such that 4^k + 21 is prime.

Terms

a(0) =2a(1) =4a(2) =8a(3) =22a(4) =24a(5) =26a(6) =98a(7) =228a(8) =246a(9) =808a(10) =1556a(11) =1792a(12) =1978a(13) =3424a(14) =69738a(15) =108376a(16) =169584