a(n) = ((4 + sqrt 6)^n + (4 - sqrt 6)^n)/2.

Terms

a(0) =1a(1) =4a(2) =22a(3) =136a(4) =868a(5) =5584a(6) =35992a(7) =232096a(8) =1496848a(9) =9653824a(10) =62262112a(11) =401558656a(12) =2589848128a(13) =16703198464a(14) =107727106432a(15) =694784866816