a(n) = 6*4^n + 1.

Terms

a(0) =7a(1) =25a(2) =97a(3) =385a(4) =1537a(5) =6145a(6) =24577a(7) =98305a(8) =393217a(9) =1572865a(10) =6291457a(11) =25165825a(12) =100663297a(13) =402653185a(14) =1610612737a(15) =6442450945a(16) =25769803777a(17) =103079215105a(18) =412316860417