a(n) = Lucas(3*n) - Fibonacci(n).

Terms

a(0) =2a(1) =3a(2) =17a(3) =74a(4) =319a(5) =1359a(6) =5770a(7) =24463a(8) =103661a(9) =439170a(10) =1860443a(11) =7881107a(12) =33385138a(13) =141422091a(14) =599074201a(15) =2537720026a(16) =10749956135a(17) =45537547527a(18) =192900151034a(19) =817138159415