a(n) = ((1 + 3*sqrt(2))^n + (1 - 3*sqrt(2))^n)/2.

Terms

a(0) =1a(1) =1a(2) =19a(3) =55a(4) =433a(5) =1801a(6) =10963a(7) =52543a(8) =291457a(9) =1476145a(10) =7907059a(11) =40908583a(12) =216237169a(13) =1127920249a(14) =5931872371a(15) =31038388975a(16) =162918608257a(17) =853489829089