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

Terms

a(0) =1a(1) =2a(2) =7a(3) =26a(4) =97a(5) =362a(6) =1351a(7) =5042a(8) =18817a(9) =70226a(10) =262087a(11) =978122a(12) =3650401a(13) =13623482a(14) =50843527a(15) =189750626a(16) =708158977a(17) =2642885282a(18) =9863382151a(19) =36810643322a(20) =137379191137a(21) =512706121226