a(n) = Sum_{k=0..n} 3^(k-1)*Fibonacci(k).

Terms

a(0) =0a(1) =1a(2) =4a(3) =22a(4) =103a(5) =508a(6) =2452a(7) =11929a(8) =57856a(9) =280930a(10) =1363495a(11) =6618856a(12) =32128024a(13) =155953777a(14) =757013548a(15) =3674624638a(16) =17836995847a(17) =86582609284a(18) =420280790476