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

Terms

a(0) =1a(1) =2a(2) =10a(3) =44a(4) =196a(5) =872a(6) =3880a(7) =17264a(8) =76816a(9) =341792a(10) =1520800a(11) =6766784a(12) =30108736a(13) =133968512a(14) =596091520a(15) =2652303104a(16) =11801395456a(17) =52510188032a(18) =233643543040