a(n) = ((3 + sqrt(7))^n + (3 - sqrt(7))^n)/2.

Terms

a(0) =1a(1) =3a(2) =16a(3) =90a(4) =508a(5) =2868a(6) =16192a(7) =91416a(8) =516112a(9) =2913840a(10) =16450816a(11) =92877216a(12) =524361664a(13) =2960415552a(14) =16713769984a(15) =94361788800a(16) =532743192832