a(n) = 5*4^n+1.

Terms

a(0) =6a(1) =21a(2) =81a(3) =321a(4) =1281a(5) =5121a(6) =20481a(7) =81921a(8) =327681a(9) =1310721a(10) =5242881a(11) =20971521a(12) =83886081a(13) =335544321a(14) =1342177281a(15) =5368709121a(16) =21474836481a(17) =85899345921a(18) =343597383681