a(n) = A047848(3,n).

Terms

a(0) =1a(1) =2a(2) =8a(3) =44a(4) =260a(5) =1556a(6) =9332a(7) =55988a(8) =335924a(9) =2015540a(10) =12093236a(11) =72559412a(12) =435356468a(13) =2612138804a(14) =15672832820a(15) =94036996916a(16) =564221981492