a(0) = 1, a(1) = 1, and a(n) = 4*a(n-1) + a(n-2) for n >= 2.

Terms

a(0) =1a(1) =1a(2) =5a(3) =21a(4) =89a(5) =377a(6) =1597a(7) =6765a(8) =28657a(9) =121393a(10) =514229a(11) =2178309a(12) =9227465a(13) =39088169a(14) =165580141a(15) =701408733a(16) =2971215073a(17) =12586269025a(18) =53316291173a(19) =225851433717a(20) =956722026041