a(n) = (3*3^n + (-5)^n)/4.

Terms

a(0) =1a(1) =1a(2) =13a(3) =-11a(4) =217a(5) =-599a(6) =4453a(7) =-17891a(8) =102577a(9) =-473519a(10) =2485693a(11) =-12074171a(12) =61433737a(13) =-303980039a(14) =1529466133a(15) =-7618632851a(16) =38179257697a(17) =-190638008159a(18) =953964881773