3^n+3n4^(n-1).

Terms

a(0) =1a(1) =6a(2) =33a(3) =171a(4) =849a(5) =4083a(6) =19161a(7) =88203a(8) =399777a(9) =1789155a(10) =7923369a(11) =34780155a(12) =151526385a(13) =655905747a(14) =2823355257a(15) =12093944427a(16) =51582654273a(17) =219172472259a(18) =928100356425