a(n) = Fibonacci(3*n + 1).

Terms

a(0) =1a(1) =3a(2) =13a(3) =55a(4) =233a(5) =987a(6) =4181a(7) =17711a(8) =75025a(9) =317811a(10) =1346269a(11) =5702887a(12) =24157817a(13) =102334155a(14) =433494437a(15) =1836311903a(16) =7778742049a(17) =32951280099a(18) =139583862445a(19) =591286729879