a(n) = 4*a(n-1) + 3*a(n-2), a(0)=1, a(1)=6.

Terms

a(0) =1a(1) =6a(2) =27a(3) =126a(4) =585a(5) =2718a(6) =12627a(7) =58662a(8) =272529a(9) =1266102a(10) =5881995a(11) =27326286a(12) =126951129a(13) =589783374a(14) =2739986883a(15) =12729297654a(16) =59137151265a(17) =274736498022