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

Terms

a(0) =1a(1) =6a(2) =33a(3) =180a(4) =981a(5) =5346a(6) =29133a(7) =158760a(8) =865161a(9) =4714686a(10) =25692633a(11) =140011740a(12) =762992541a(13) =4157920026a(14) =22658542533a(15) =123477495120a(16) =672889343121