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

Terms

a(0) =1a(1) =2a(2) =7a(3) =24a(4) =82a(5) =280a(6) =956a(7) =3264a(8) =11144a(9) =38048a(10) =129904a(11) =443520a(12) =1514272a(13) =5170048a(14) =17651648a(15) =60266496a(16) =205762688a(17) =702517760a(18) =2398545664a(19) =8189147136a(20) =27959497216a(21) =95459694592a(22) =325919783936