a(n) = 4*a(n-1) - a(n-2), a(0)=1, a(1)=6.

Terms

a(0) =1a(1) =6a(2) =23a(3) =86a(4) =321a(5) =1198a(6) =4471a(7) =16686a(8) =62273a(9) =232406a(10) =867351a(11) =3236998a(12) =12080641a(13) =45085566a(14) =168261623a(15) =627960926a(16) =2343582081a(17) =8746367398a(18) =32641887511a(19) =121821182646a(20) =454642843073