a(0) = 1, a(1) = 5, a(n) = 4*a(n-1) - a(n-2).

Terms

a(0) =1a(1) =5a(2) =19a(3) =71a(4) =265a(5) =989a(6) =3691a(7) =13775a(8) =51409a(9) =191861a(10) =716035a(11) =2672279a(12) =9973081a(13) =37220045a(14) =138907099a(15) =518408351a(16) =1934726305a(17) =7220496869a(18) =26947261171a(19) =100568547815a(20) =375326930089